I row an Adirondack Guide boat and really love it; I retire to the rowing machine when its cold. I think its a great upper body and thigh compliment to bicycling.
I am not convinced that a kayak could be effectively modified to row. First, you sit lower in a kayak than in a rowing boat, and second kayaks tend to be a bit directionally unstable without an added keel or skeg. Could be done, I suppose, but the weight and performance penalty might leave you disappointed. I used to build canoes and kayaks for a major company. Nearly all of our hybrid boats were inefficient compromises that ended up being the worst of both worlds.
Definitely check out the ag sector for work - it's very rewarding.