Almot wrote:
The only exception are Lithiums.
THIS is the only answer which really matches the OPs real request.
VERY expensive though.
Since the battery will be attached to the boat......weight really IS an important consideration as the battery weight might ensure that the boat goes TO THE BOTTOM if it ever tips over.
A conventional is not a good choice because the electrolyte will be contaminated if it is immersed in water. An AGM would not have that problem.
I think what he really needs is a small LI type in the boat and a good AGM in the trailer.
Trying to use a single battery for both is, I think, a big mistake.