There are/were two manual Raritan toilets desginated as II...one of them--the PHII doesn't have a wet/dry flush LEVER, it has a knob that you turn. So I'm guessing you have a Compact II, which was discontinued several years ago, btw. Here's the link to the owners manual:
Raritan Compact II Owners Manual It doesn't spell out the direction of the wet/dry lever to bring in flush water or close it off...but it should be pretty obvious which way brings in water when you pump the toilet in each position and which way doesn't. Make note of it so you can tell your guests.
However--as you almost found out the hard way--leaving the seacock open and relying on the wet/dry valve to keep water OUTside the boat from finding its own level INside the boat when no one is aboard has sunk many a boat in its slip...you were lucky you hadn't stayed away any longer. ALWAYS close ALL the seacocks last thing before you leave.
But that's not all you need to do. Whoever installed the toilet failed to install a vented loop in the toilet intake line...see "FIGURE 2 Below Waterline Installation" at the bottom of page 3 in the owners manual for the correct location...which is NOT in the line between the thru-hull and the pump, but between the pump and the bowl. It has to be at least 6-8" above waterline AT ANY ANGLE OF HEEL, not just when the boat is at rest, which on most sailboats puts it 2-3 FEET above the bowl see the drawing and also the photo below (you only need the small one that's tucked under the one that would go in the discharge line if your toilet were plumbed to flush directly overboard.) That means you'll replace the short piece of hose that connects the pump to the bowl now.
You seem to be kinda new at marine toilets and also the holding tank you surely must have in your waters...so I suggest you check out the link in my signature here...it goes to a comprehensive "marine toilets and holding tanks 101" manual that will help you learn how to prevent problems instead of solving 'em...and I'm always glad to answer questions too. So anything you don't get about what I've said here, ASK...you'll get plenty of help.