Most of the major military antenna suppliers will have marine qualified products. You will need to decide on your own requirements for gain versus coverage - usually hemispherical coverage (or more, due to pitch and roll) is required and that tends to put limits on the gain unless you decide on an active solution ($).
Most "boats" will have a large mast structure that tends to 'get in the way'. You need to decide if you need to use a multiple antenna system, accept shadowing, or find a practical way to install your antenna at the very top of the mast (usually impractical).
There much more we could talk about - but perhaps off-line.
Probably won't go down the active route because of weight and cost implications. I'm only at the budget stage at the moment and so looking for ball park figures.
Thanks for your help and the links, which at first glance seem very useful.
