I consider myself a porta potty expert... sorry, Peggy.
My advice on the porta potti...
Don't pee in it. Pee overboard. Pee takes up too much space. The only person who gets to pee in mine is my wife, and then only at night when it is cold.
Get the 5-gallon model... better safe than sorry.
You would be crazy to empty a porta potty into a bucket and then keep the bucket on board. It will be gross, it may eventually spill, and you will regret it.
I use the blue tablets or the blue liquid for my porta potty, and they work pretty well, but the best thing I ever did was to start adding some bleach to the fresh water in the porta potty. It keeps the smell down a lot and just has a way of helping to keep everything smelling more sanitary.
Empty the porta potty as soon as you can when you get off the water -- almost every rest stop on the highway has a dumping station. Just remember to find a way to suppress your gag reflex when you start emptying it.
Read this next part carefully...
1. DO NOT come home and simply put the full porta potty in your basement and leave it there until your next trip.
2. If you come home with a full porta potty, DO NOT let your wife watch while you dump it out in the toilet.
3. Clean the porta potty after you get home (with a hose and one of those rubber hazmat suits if possible) or else you won't be able to stand the stench inside the boat the next time you use it.
Good luck, my friend.