I am a guy who likes to go nude at beaches, resorts, spas, and other friendly nudists venues. After 10 + years of doing this, you see a lot of penises, and eventually, you see very big ones, and hard ons. Like everyone here has said, its really bad etiquette to get a hard on, and if it happens, you should cover up, discreetly get some privacy or whatever. Sometimes it doesn't work out that way - here are a few things I've seen over the years:
At a semi out door spa at a clothes free weekend farm about 8 years ago, this guy came out of a sauna very erect. He headed straight to the showers which took care of it
A guy with a very big cock lazily wandering the beach at Little Beach on Maui. He wasn't all the way hard, but he was really big, heavy floppy. I think a lot of people looked.
Saw a guy with a hard on while laying on his back at some nude hot springs
Finally, was in Vegas and in a spa and saw a dude my size come out of one of the private massage rooms with his robe wide open and a massive erection. Before you think 'what went on in there', he came out alone, the therapist was obviously very professional, so the bone must have happened afterwards. They let you relax after the message as long as you want, so he must of boned up then. Anyway, he just came out, robe open, and cock fully extended and swaying back n forth as he went to the showers.
Go figure!