Imagine if you were to race back to your boathouse for shelter from bad weather and you can't get in it because somebody is already in it for shelter. Just sayin.
If so, you tell them to leave, or you squeeze in as much as possible, tie off and get in their boat or on your dock. Or go in your house.
For anyone who doesn't want people fishing around or getting into "their" boat house-slips, put up a screen or door over it that goes to the water. Put up a sign that politely says, "Please don't fish around here because we have children who swim and play here" or something like that.
Marketing isn't just about products. PUtting up a sign that says KEEP AWAY, PRIVATE will just cause people to ignore it. One that mentions children playing around there might be enough for an angler to troll on past.
Throwing some old box springs under the dock and boathouse slips also can discourage them, of course.