I rarely get too warm when I am kayaking. Hiking with your boat in the hot sun is another issue though. I would much rather sweat my ass off on the way to the put in than be shivering while I am boating, and trying to avoid every wave because I am so cold. Boating while cold is not fun, period.
I wear a drysuit most of the time I boat, if you don't wear a whole lot under it, you actually don't get that hot.
However, you said you are a big guy, you may not get as cold as easily as me. You might be ok with a drytop and shorts, unless you swim a lot.
I say if money is not an issue, get the drysuit, then buy a cheap shorty drytop or something for when it is really hot.