I didn't even think about Pyrous Mountain...So realistically the beach would be a dark gray (pumice color) or black like in hawaii. Now with that in mind, you could always do the beach like hawaii. Make it black and make it different height gradients because it could be lava flows.
Kinda like this image. It could be black sand beach down by the academy but north and south of that I think it would be really cool to have lava flows, and you could make it steamy* from the lava hitting the water. This could follow the story-line as in the lava would be flowing in rivers while team meteor is messing with the volcano and then when the pulse machine is destroyed it stops and all of it on the beach cools down (at least at the south end of the beach, the north could just be continuous).
*opportunity for the use of defog. As in, you could not cross the steam because it would be scalding hot so you have to use defog to clear it so you can get by.