I had the same idea and decided to try loading up my GTR215 with some ballast using a pair of 250lb Sumo-Sacs I had swapped out of my wake boat. I tossed one bag in each foot well and then used a ratchet strap hooked on both sides over the top of the seat to hold the bags from sliding around too much. I estimate I only added about 400 lbs as the bags did not fill to full capacity because they were wedged down in the footwells. Combined with a 275lb driver, it put out a wake that looks big enough for a small rider to surf behind, but we have not had a chance to try it yet with an actual surfer.
Intrigued by the idea, I proceeded to build a wedge that will pull the back of a pwc down similar to what Malibu puts on their wake boats. The first try was a little large and slowed the PWC down too much (10 MPH max speed at nearly full throttle). It did pull the back 1/3 of the PWC completely under water, so that is about the most displacement the boat can make. The wake was maybe large enough to surf a small rider,. I might need to put some ballast in the front to even things out when using the back wedge as the nose was pretty high.
You can find some videos of people surfing behind PWCs on Youtube. The two I saw just had a rider and a passenger on the PWC without any other ballast. They leaned to one side to shape thewave a bit.