Assuming you don't have access to a bathroom on the boat, there are a few options for how to relieve yourself while out on the water. One is to use a portable toilet, if your boat has one. If not, you can do what's called catching a bucket. This involves going to the back of the boat and using a small container or bucket to urinate into. Make sure to aim well and away from any people or areas of the boat that you don't want to get wet! You can also try squatting over the edge of the boat, but be careful not to fall in. Another option is to wait until nightfall and then do your business overboard (again, being careful not splash anyone or anything). Whichever method you choose, just be sure to be respectful of others onboard and clean up after yourself when you're done.