Wait until you have an otherwise trivial mob kill you by simply pushing you into the water and hitting you until you drown while you're completely helpless and can't fight or swim back.

Water interaction in general is completely broken in Valheim. Always think twice about entering the water, and then pause and think again, and check your stamina. If it looks like you have plenty of stamina to get across that stream, you probably don't, so always swim for rocks and shallow spots.