You have to be very very very careful when it comes to stickers on boats. 

At the Albacore internationals they use stickers for the tally number and the sponsers logo, these stickers were very good and stuck really well maybe to well. No one had told anyone how to take the stickers off so everyone pulled them off when they got home and found that they were that sticky that they took the varnish, paint and in some cases the gel coat off the boats.  One of the guys down at my club is a boat builder and uses his boats to get business at albacore events where allot of the boats are made of wood with paint or all over varnish and he took his sticker off and it took the varnish off in 2 very big patches on both sides.  My boat is grp and it took very small un-noticable piece of gel coat off.

The best stickers that I ever had to put on a boat were for the Gill Abersoch Dinghy Week last year.  there were very sticky but at the same time came off easily it may be worth contacting them and asking them who supplies there stickers

I would be very careful when selecting stickers for boats.

true but sometime not even that works.  If you look at some of the albacores around some of them are very carefuls as to how close they get with anything that produces hot air or may damage the varnish.