Not much of a contest here in my opinion, between being able to stay submerged until you need to copy the captain keyword, to the versatility of being able to copy Longship, defense, offense, etc etc.
Not completely sure where to rank, but combined with submerging, extra actions somewhere, speed boosts, and even copying it with Behemoth, this thing can be devastating on a level not seen with any other monsters.
Likely underrated, this is one of the only ways to give a sea monster the move-and-shoot in one turn. Also keep in mind it says ANY other monster, which can include those of your opponent(s).
6. Mist Walker
For those 3, as far as I’ve seen L is hardly ever fast enough to catch anything, so speed that cannot be increased with helmsmen is very important for these creatures.
Still need to use it.