Since they limited respawning leeches this really is less of an issue as long as you clear the area first.

So you don't want to land there first typically... it isn't too bad when you are past the iron age and going back but the first time to get iron you want to avoid landing in the swamp first.

But once you go in and clear out the mobs it is a whole different story... it seems "fairly safe" to leave a boat along a swamp shore now as long as it has been cleared first.