The shores near plains tend to have lots of places to easily beach your boat, especially when fog creeps in and you can't see where you are navigating. I suggest staying as far away from Plains as possible to avoid that.

I would not worry overmuch about the deathsquitos flying over the ocean to get you; as long as you're a decent distance away, they'll not aggro to you.

Valheim terrain near Swamp (in the shallows) tends to show as "Plains" quite often, but if it's all under water, no need to worry.