They didn't bother us when we did it but they are apparently a common problem so the use of Deet is probably a good idea. As for restrooms, there are facilities when you arrive at the place to pick up your equipment and you return to this stop just before leaving. In addition you will stop for lunch and they have facilities there as well.
The biting flies in Belize are a real problem. Try skin so soft from avon, a lady on our mangrove tour used it and it seemed to work well. On this tour, they actually have a fly swatter per seat on the boat. You would not believe how vigilent these flies were.
It depends on what time of year you are there for the bugs. We went end of February and had none. They have a rainy season, that is when bugs are bad. We took bug spray, and had no problem. They have restrooms right were the buses park before you make the 45 min walk to start of river. When you get done with the tubing, you are a very short walk back to parking and restrooms.
Oh and be careful what you bring back, next morning husband was shaking his swim suit and a huge bug fell out. It was funky looking. Dont know when he picked up bug, we changed in restrooms after the tubing. Glad it was him and not me
We were there two years ago in July. I used sun screen with bug replent and did fine. My friend did not and gotten eaten alive on his back. He was so uncomfortable he missed dinner that night on the ship.