fnxrak's Undrowning Glitches and Spots topic has the most comprehensive documentation of the undrowning glitches that I've seen. Post #18 includes notes on using the Vortex as a Submarine. The sub-vortex trick works when you use a vortex on a unique jump and land in water. The jump into the lake above the dam is a natural for this glitch but limited to the upper dam lake. There are a few other natural spots to do the glitch but they're kind of tricky. Anyway, when the vortex lands in water after a USJ the slow-mo camera angle will persist until CJ is so far out of range that the water no longer supports the vortex. It's sink below the surface until it hit's bottom and the camera reverts to normal. The Vortex will continue is it's semi-buoyant flght mod and can be flown around under the sea like a mini-sub. It's one of the funner ways to explore under the sea as you don't need to worry as much about soft-spots, large rocks, and unclimbable hills. Try backing up the steep hills, btw. Sometimes it helps.
There are two vehicles placed under the water to support underwater exploration and the underwater USJ. I think they are unlocked by boat and bike school medals. Both are located in the Panopticon. An Infernus marks the large deep zone south of the mill, and an NRG is hidden under the bridge near the airport. Bikes don't sink, as you've noticed. You can get a BMX under the Verona Beach pier if you approach it correctly, but motorcycles aren't usually used for underwater travel, so we put one in.
IIRC, if you've got GSF in the other cities you still can't recruit them after Green Sabre, maybe. I'm not sure anymore so let me know how it turns out.
I seldom find GSF to be the least bit useful for anything so I tend to ignore them. The Coach or Bus might be able to hold a full crew of CJ+7.