Do you live in a house or on a boat? Either way, cats are territorial and uncomfortable when in new territory. Might take some time before your place becomes her place.
Also, I think that cats that are used to being outside find being inside pretty boring. They want to go where they are more entertained. Cats who are raised indoors don't know what they are missing and are content to stay inside.
Finally, if the cat is a stray, it may have a limited tolerance for human company. Add this to being away from it's usual territory and it may need to go where it is more comfortable to de-stress.
You don't mention how old the cat is, but like older people, older cats can have a harder time adapting to new situations.
They say that if you have a cat in your family and you move, it takes about a month for the cat to accept that your new house is its new house.
Cats can be quite comfortable on boats. They are unlikely to intentionally jump off into the water. However, they can misjudge jumping to/from the dock or other parts of the boat if things are moving (just like people). When they miss, the smooth hard surfaces on most boats can make it hard for a cat to get a grip (claws aren't much good on fiberglass or metal surfaces) and it can fall off.
We had family friends that took their cats when cruising on their boat. One fell off the boat one night. Luckily, they heard the splash and where able to fish it out before it got carried away by the current. If you cruise with a cat, you might want to make sure you have a landing net with a long handle.