Sims Cannot Get Out Houseboat

lightsqber​  To be clear, the sim in the screenshots is trying to walk to/along the dock?  If so, replacing the houseboat likely wouldn't help—the problem is probably the dock.

If you use mods, or are willing to try some, add NRaas MasterController.  Click anywhere on the ground of the lot that holds the dock and select NRaas > MC > Object Stats > Not Inventory, and flush any item with the word jig in it.  These are placeholders for interactions that make other sims walk around the sim(s) doing the action, and they're supposed to be deleted when the action ends, but sometimes that doesn't happen, for example if the sim is reset, or you quit and reload and the action immediately gets canceled.  Point is, Object Stats flushes the item(s) you select.

If this doesn't help, walk the sim a square or two at a time towards where you want them to go, and see where they stop.  Then go at that area with a sledgehammer.  If you find you can't delete certain tiles, use MC to look for hidden objects there too.  This time, click the spot and select NRaas > MC: [with the colon, not the bullseye] > Radius Purge, put in a low number, and see what's in the immediate area that doesn't belong there.

Note:  The two MC menu options are distinct.  One brings up the usual menu with options like Reset Lot and Object Info [centers the camera on whatever object you choose]; the other is for the Radius Add to Inventory and Radius Purge commands only.

Finally, one time this happened to me, with a regular house as opposed to a houseboat, and I had to replace the front door.  This was despite the fact that the sims could leave the house to go outside.  They just couldn't go to work or school while already standing outside.  I didn't get it, and still don't, but the problem was gone just like that.