I would say try some harsh chemicals but it might ruin the finish of the boat.
Get some Windex or go to the paint section at Wally World and look for Goo Gone and a window scraper (the razor type, if you don't have one), it's about less than $5 all together...
The Goo Gone or Windex should get the sticky stuff off... scraping with a window scraper should make it easier on your fingernails.
OR use just use Windex and the scraper, spray it and scrape it at the same time.
This is what I use to take window tint off cars