Summoned boats are unaffected by the Rotation tag.

I tried adding the NBT tag {Rotation:[0f,270f]} to a summon command for boats expecting it to work. It didn't, and summoned boats appear to always face south.

How to reproduce

Put the command /summon minecraft:boat ~ ~1 ~ {Rotation:[0f,270f]} into your chat window or a command block and run it.