IMHO I'm not sure a boat
is a place for a safe, I think you might be better off making a hiding place.
My reasoning is that a safe is a delay mechanism designed to delay the burgler long enough to get caught. On a boat
anchored a couple of hundred metres off shore there isn't much urgency as someone coming can be easily identified in enough time to escape.
It is also a red flag that you have something of value and may bring about a return visit while your onboard.
A reasonable quality safe will be more easily removed than opened so whatever you fix it to will likely be damaged and that may be worth more than the contents. The only things in a boat strong enough to attach to aren't things you want a burglar to cut out.
If you hide it and it's found, it's gone but without additional damage.