No reason to uses anything other than the double side that is already on the Hydroturf. BUT... I would recommend cleaning the boat very well... and then wipe down the boat with Acetone (in sections) just before you stick it down. (to make sure there's no wax)
FYI... I've never had a "Stick" failure on any of my toys, and the 3m stuff that hydroturf uses. AND, it's WAY easier than using contact cement, like in the old days.