You haven't been reading the other posts of folks just like you. Dropped anchor in the swamp. Nope, bad idea. Now you know. Approach swamp on foot, always.

You can also stand on the fallen indestructible trees that are everywhere and chop him down with your axe, be under him and aim for left front leg, its left, not yours, or jump on a crypt and put a new portal there.
One portal stays near the swamp, leave it for now, the next one is a roamer that you take from crypt to crypt.

The whole thing about boats in this game is folks use them instead of exploring on foot and it leads to many problems. You can't leave the boat in water in swamp as everything wants to destroy it. And if your rng is bad, you get the abomination welcoming committee. Same with plains, just don't do it.
Live and learn.