On the ones I have, I don't see how the suction cup will hold by itself. They seemed designed to have that portion glued onto the boat, then the light attached as needed.
I use the suction cup for the stern light, on top of the outboard, and a glue on detachable mount for the bow nav light. Both are LED, and work well, and the batteries last forever. The suction cup will not work on the boat itself, they simply will not stick. Find them here
I used zip ties and wrap them around the bow and around a tiny hole in the very top of the transom mount for the stern. If you need to take them off, you simply cut the ties but I could never get the lights to stand up on their own with just the velcro.
They come in a variety of colors and come in handy since you can clip them to the rail that runs around the boat. They are small and I still carry a flashlight to let bigger boats know I'm around when they're coming up on me just to be sure. Seems to satisfy the law, though. They're rated for SCUBA depths so they're durable enough for me.
