if it was a bad thing, there would be a LOT fewer guys backing their boats into the water. :yes:
an engine that is cranking over should have no trouble pushing water out the back end. it's just water. :thumbsup:
buuuuuuut, if you're SO submerged that water is above the headers (essentially submerging the whole exhaust system), then water could hypothetically fill the whole pipe and get into your block. don't do that. :no:
on an unrelated note, I now want an exhaust flapper thingie. just because. :D