I used to fish grapevine quite a bit from my Hobie Revolution and trolled often. If I was moving to a new spot I always trolled something. Rat-l-traps work, but if you stop to reel in one rod the Rat-l-trap on the other will sink and get snagged. I preferred floating crankbaits. The original Rapala shad rap seemed to work well. Sometimes I would put a shallow running lure on the side closest to shore and a deep diver on the other side and then pedal along in 15-20 feet of water. A fish finder/GPS unit really helps - just follow along the shore at whatever depth you are targeting.
I use a very similar approach. I usually troll two lipped crankbaits (usually Bombers), one designed to run 10-12 feet and the other a little deeper and use the depth finder to try to keep both baits within a few feet of the bottom. That technique produced my two largest largemouth on Grapevine.