The most effective method of affixing registration numbers to a PVC inflatable boat is to put the registration numbers on a 3 inch high board and then hang the board over the side of the boat as near to the bow as possible. The perimeter rope handles that are included on all Newport Vessels inflatable boats, work as a great attachment point. USCG rules say that the vessels registration numbers must be visible at all times (not permanently attached).
If you're insistent on permanently attaching letters to your boats, we do not recommend using stickers. These almost always crack and fall off within a year. Instead, we recommend using a stencil and then using either a permanent magic marker (which works great) or using Krylon spray paint with very light pre-coat primer. The best spray paint is Krylon Fusion for Plastic (2326).