Water sloshing in my stomach during the run portion??

When you exercise, you are less efficient at moving things from the stomache to the intestines.  So if you drink too much too fast, you get the sloshing problem.  And when you stop, all the fluid gets absorbed quickly.

I would suggest training as you race - if you plan to drink on the race, drink on the training runs as well. Or keep yourself to smaller drinks of water at a time, so there is less fluid to move.

I used to have the same problem, here's what I do now:

1.  Go into the race well hydrated, drinking extra (or making sure not to skimp) in the days before.
2.  Get in a good bottle on race morning, with enough time to use the potty.
3.  Drink moderately on the bike, enough to go with my gels and to thirst.
4.  Only drink to thirst on the run (or a whole cup with my gel, thirst otherwise).

I end up a little dehydrated, but generally a LOT better off than before.  This just works for me, anyway.  Good luck!

Thanks everybody for  the advice!

This is a good point, and my advice really only relates to my sprint, Oly and a few half-marathons.  Beyond that, like with your planned HIM, you will need to take on more water probably.  Just be slow about it.  You still probably don't need a cup of water at each station on the run.  Maybe considering bringing a bottle so you can sip?