You will definately get 'forests' of Kelp
The calculation to place them is pretty simple. The chance is weighed by the plots' distance from the map edge and temperature. For coasts that is all I do, so if the plot is pretty close to the edge (say 0.8) and the plot is the 'jungle' climate (so about 0.85+) you'll have an 82.5% chance of kelp appearing.
Once you get away from the coast it gets a little more complicated. First, ocean temperature itself is affected by the depth of the ocean (deeper = cooler). Second, kelp placement in the ocean is limited by depth (the 25% that is most shallow). The final chance is similar to the coast chance with more emphasis on temperature, so it's basically the average of the edge weight and the double the temperature (technically its the coast calculation + the temperature / 2

).
So what you get is a pretty even random scattering of kelp along the coast with more in the warm areas. The ocean kelp follows the heightmap that you can't see because it's underwater so it really just get dropped in like a forest would be if it were land so you can almost see the continuation of the continental plates though the water. Because of the edge weight, kelp density drops as you near the center of the map, so inland seas or similar waterways may have some kelp but it won't be very thick. Since the whole thing is simply a random chance per plot the overall spread is similar to the way ice is placed near the pole(s) with some clumping and some scattered bit here and there. It definately helps convey the climate in the water, also it is dependant on the tundra map option, you'll never have kelp in water that is cold so if you run with massive (or is it very high?) tundra you'll have very little kelp overall. It is not tied to the jungle temperature directly, so having no jungles will yield the same kelp as you will see with massive jungles.
Right now it's just a matter of tuning the base calculation to get the right amount of kelp, but we'll have to let you see it before we decide which way to move it. When I send you the test version I'll give you instructions on where to make any adjustments so you can play around with it yourself if you want, then once you're happy you just need to let me know what values you used.