Duckster,
In your situation, a full sink would be what I'd use.
Since you will be fishing from a boat rather than the shore, getting down and staying down are important. The 'belly' of the line that everyone complains about would be a 'plus' in this situation, keeping the fly deeper longer on each cast. For this reason, you want a regular old sinking line, not one of the fancy new 'density compensated' lines (I really like these for shoreline lake fising, since the 'straight line' keeps me out of the rocks).
I'd also get the fastest sinking line I could throw. You can contol depth by countdown and retrieve rate, and the faster the line sinks the more casts you'll be able to make. This increases the amout of time the fly spends in the strike zone, and thus increases the number of fish you'll catch.
I catch way more trout on a sinking lie in stillwaters than I do with a floating line, simply because it gives you positive depth control. Seems like I always have at least one or two rods rigged with sinking lines in the boat, and they tend to be the ones that catch the majority of the bigger fish.
Good Luck!
Buddy
It Just Doesn't Matter....