Good choice for heat-conductive coil submerged in sea water
My advice is to use copper and protect it with a sacrificial anode.  Copper would be the choice in a non corrosive environment and if you can protect it go for it.

However, if the coils will contact parts of the boat that are important than the coils should be aluminum.  Copper would cause the other metal parts to corrode faster.

Why not use rubber or plastic, something like the pipes used in drip irrigation?