After finding the actual statute this is my understanding as well. Looks like maybe you only need it if the tank you transport shad from the lake is more than 30 gal. Since you can`t transport them on the boat anyway this does not seem like an issue for on-board bait tanks. The guidance documents by TPWD are more confusing than the actual statues.

http://txrules.elaws.us/rule/title31_chapter57_sec.57.385

I don't know The way I read it is you need a permit if your tank is 30 gallons or more. Look at the second bullet point and tell me how you read it. My tank is 40 gallons but only plan on putting 20 gallons in it. It does not say anything about transportating that I see.

I was looking at the part that says
The volume limit does not apply when shad are possessed:
(A) on the waterbody from which the shad were taken; or
(B) by a licensed fishing guide.

I think it might apply if you want to keep them day to day. But I don't think you can transport them on a boat even if you use city water.

I was thinking to install a threaded bulkhead fitting so it would overflow at 30 gal if I had to. Like an ice chest drain only near the top.