Towels, jackets, dog's leash, a lamp - the suction cup with with towel hooks can hold very different things.
Does your trip take you to the sea? No problem. The salty sea breeze can't harm the rubber disc. The hooks are made of stainless steel so this part is also immune to salt atmosphere. The body is made of aluminium, which again can resist a lot of this salty sea breeze. All other parts are galvanized for an extra layer of protection.
Please note though that it's not intended for a sailing trip.