Just to clarify here is how to reduce signal interference.
1. Keep power, comm, and signal Wires separated by at least 12 inches If they must cross do it at 90 degrees.
2. Fold then coil excess signal wire.
3. Make sure Fuse box is still in good shape or run clean power off cranking battery with 3 amp Fuse.
4. Ground the negative terminal cables on the cranking battery to the trolling battery with 18 ga wire with 1amp inline Fuse, 2amp is OK if you can not find the 1amp.
5. Verify the engine is grounded, continuity between negative terminal and skeg.
6. Verify the trolling motor is grounded. Most trolling motors 2013 and later are grounded with Fuse inhead on motor. Same test as engine, negative plug to skeg. the f your TM is not grounded, run an 18 gauge wire to the skeg or strnsducer band and ground it to the heat ve buzz and the accessory ground with a 1 amp Fuse between the later two.
7. Ground trolling motor negative to accessory negative up front, I.e TM negatively be plug to negative ground on livewell timer switch or trim switch. Use 18 ga wire with one amp fuse.
8. Add signal choke to trolling motor power wires if is has battery maximizing. Works best at the foot control, but still helps at the socket.
9. Turn down the signal power to the transducer.

Just a few "changes" or "comments" on this:

Good catches by Mr Jones.

Introducing new signal ground paths rather blindly across high power connections can have potentially damaging results.

This all started when I upgraded by 20 year old trolling motor with a scope to a new state of the art trolling motor with high definition side imaging sonar system. The old boat had a 12/24 trolling motor power distribution system with the bus bar up front controlled by a toggle switch. The new trolling motor is a 24 volt variable steed power management system of the same maximum amperage and pounds force. Thinking it would be easy.
Right from the get go there was controversy and confusion, plus I've only had bad experiences with local marine shops in the past. But that is different topic. The existing breaker was undersized for the wires, no big deal, change to breaker to match the trolling motor maximum current and the wire amperage draw. Then the problems started.
The trolling motor manufacture installation instruction did not address grounding the batteries one way or the other, but the original trolling motor manufacture required a common ground wire and had very strong warning if you did not. The new trolling motor's sister company that manufacture the HDSI sonar equipment installation instruction said to remove the ground wire between the batteries if using the USB2 build in transducer. There was no guidance provide in the sonar installation instructions if you mount a transducer to the trolling motor skeg, but the manufacture on its technical Q&A website pages recommends a fused small conductor ground be added if you have a noise issue. Which is it? It should be an easy answer, but it is all over the place.
Did I need a ground wire between the trolling battery and the cranking battery neutral terminals or not? This is why I had my previous post, a 'shout out' on this forum, to obtain a reality check. I did not expect a slap down, maybe a fishing forum is not place for a technical. But that's okay, it motivated me to find the correct answer to the question. After all is done I agree 100% that anyone that willy-nilly makes alterations to the wiring on a boat should be prepared for some potentially serious consequences including fire, corrosion, and/or personal injuries including burns, electrocution, or drowning electrocution. Unprofessional or otherwise modifications could also affect insurance coverage and/or lead to legal actions against the owner or the shop in civil court if certain minimum standards are not followed. In some cases minimum standards are not adequate in a juries eyes if that is the point of failure. Fine, the slap down was warranted, and fishing forum should probably stick to fishing and not boat design issues if life safety could be a concern.
Nevertheless, the answer is yes, the common ground design standard, as it is called, is actually required by federal law and has been since 1991. The requirements are defined by the American Boat and Yacht Council (ABYC) design standards and the section pertaining to AC/DC current was section E-10 and E-9 have been combined in section E-11.
1) As for the Common Battery Ground Question, the problems is, it is not very clear about trolling motor batteries because they are called storage batteries when it comes to ABYC. Now if you have an inverter or on-board charger or other form of AC ship to shore power or if you have a battery switch that mixes the two systems, then ABYC Section E-11 requires the common ground wire not less than one size small than the cranking battery conductors for life safety and corrosion prevention. I would still add the common ground to protect my family and friends, because electricity and water are unpredictable.
2) As for adding a Fused Common Ground at the bow, this is a big no-no. Grounding the cranking battery and storage battery DC systems using a neutral wire that normally carries current back to neutral could overload a wire that has not current protection. To properly ground any point it has to go all the way back to the neutral bus or motor neutral.
Lots of free advice out there, but you need to be careful who you seek council. My grandpa told be once that I should "Believe nothing I hear and only half I see", and "If you don't know what you are doing, then don't do it". I guess both are good time tested advice.
Happy Boating!

Didn't intend to give a slap down, but wanted to caution those heeding your advice. Throughout this, you've not said what mfg of trolling motor you have. The trolling motor and it's batteries are to be on a separate, isolated circuit from the outboard and are not subject to the grounding between batteries -- except when you connect the batteries with a switch to share batteries, then they become part of the outboard system. However, frequently to reduce noise or interference, the ground of the cranking is typically connected to the ground of the trolling motor. MinnKota with the US2 tranducer achieves this through a bonding wire that is connected to the ground of the control board (when the light comes on and grounds the case of the motor, setting up a shield or "shunting" the ground. The US2 is connected to the transducer ground and makes a ground connection through the depthfinder connection of the cranking battery. For this reason, trying to "share" a ground from trolling motor to cranking battery other than that provided for in the wiring of the trollingmotor US2 can result in a direct short through the transducer/depthfinder, damaging both. This is why there is a 3A fuse in the boding wire located in the head of the transducer. Motorguide has a ground strip internally that feeds ground from the trolling motor ground to the case in order to shield the motor noise. As you said, there is a lot if information on the web and not all of it is good. While it is true that shunting the ground will help relieve interference, there is ways that are unique to trolling motors and procedures to follow. It is not Generic for all trolling motors.
John