Regular strand copper sealed properly at all connectors or go ahead and pay for Marine Tinned Copper. What's the scoop.

"Marine" wire resists oxidation and corrosion better than regular stranded wire. Even sealed connections can eventually break down letting in moisture.
The other thing is that marine wire is going to be AWG as opposed to SAE which is what you are going to find at the big box and auto parts stores. SAE wire is 12-18% smaller than the same gauge of AWG and will not conduct as much before overheating.

What he said...and

Mind you I only run freshwater these days but it seems every time I need to wire something my spools of tinned wire are just short of what I need so I've run plenty of straight copper on my rigs and never had a problem.

Non insulated connectors double crimped with adhesive lined shrink tubing. getting the splice strong and water tight is more important than having the tinned wire in my opinion. If you do go tinned you can find it cheaper from industrial supply houses.

Mind you I only run freshwater these days but it seems every time I need to wire something my spools of tinned wire are just short of what I need so I've run plenty of straight copper on my rigs and never had a problem.

I like non insulated connectors double crimped with adhesive lined shrink tubing. getting the splice strong and water tight is more important than having the tinned wire in my opinion. If you do go tinned you can find it cheaper from industrial supply houses.

If you're freshwater 100% of the time, and don't want the tinned strands, at least look for the marine grade insulation. It resist the oil, gas, and other chemicals that cause other insulation to turn loose. The gauge you get ie: 6ga, 8ga, 10ga, etc, will be the same core size diameter with a difference in the number of strands inside the wire. You want the one that has more strands.
John

The insulation is another reason to use marine wire. It's is resistant to oil and gas and also will withstand high temps(105C). I have not seen any wire rated for marine use that wasn't tinned, so that's an additional bonus. Functionally, copper stranded wire will work OK for freshwater applications, but using the better stuff just reduces the likelihood of electrical gremlins popping up. There are several places online that sell marine wire are reasonable prices and by using it, you are covered for all conditions and environments you might find yourself in. If I'm doing a job for a customer, I use nothing but the good stuff and in 20 years, I have never had one come back on me. The marine environment is tough and the best is barely good enough