taping is the most sure way of keeping things flat in the water, Ive been doing it for years when my dysphoria is an issue, using both medical tape and duck tape. You have to take your time doing it though, and have a completely shaven area everywhere the tape is going to be. most of the time it can be done in about 10-15 minutes. the typical method involves taping from the top of the bikini line to between the legs, over the anus, and up the back side to the top rear of the bikini, usually with a "belt" of tape linking the front and back since the back can pop loose sooner than the front from bending and moving more. the downside is that usually means you have to remove/redo it every time you need to do #2, but if you work it right, you can make it stay completely flat with a hole through the tape in the right area to do #1 from while sitting down. Being a bit more creative and patient with the taping, I've been able to get it to where #1 and #2 are possible, but it took the better part of an hour to do and involved rolling/tucking the scrotal area in on itself to look like labia and taping it separately, then inverting the phallus and smoothing down the area with additional layers to pull it all together to the correct anatomical form. the longest I've been able to keep things in check with a single taping was about 5 days... but keep in mind that the longer the tape is on your skin, the more the glue residue will remain after removal, and odors can build up too from the skin not being able to breathe.
There's always the quicker method of just using a gaff or the control top from pantyhose that got damaged and just trim it down to fit seamlessly under the bottom, which sometimes works good for helping hide the tummy too when in a 1 piece thats not a miracle suit or one size smaller than normal (regardless of if it's skirted). From what I've gathered, natal women wear their clothes a bit looser than we do.
this was from last october fully taped. Had to be creative on the side so no taping showed under the gold buckles. I have other photos of the actual tape if anyone is so inclined, but they are too personal to show publicly.
