who exactly becomes friendly when i make peace with dwarfs?

[Gatts]

yep - the dwarven units in underground see map are all set "neutral" - or rather "friendly" from the beginning -> so no loss there...

The main difference in having or not having peace treaty with dwarfs is that the sub-marine ship will be available if you are in peace treaty and the captain won't be willing to help you if you are not in peace treaty.