The stompgrate on my Thunderjet is another grate that fits over the stock intake grate. Two rods (spring loaded) extend up through the swim deck to a "stomp"lever. When you step on the lever the two rods are pushed down and the grate is pushed down and away from the stock grate. I stomp on it a couple of times with the motor off, and the debris comes floating up. I don't know about kelp and seaweed but it works good for leaves and other junk floating in the rivers. I don't know if anyone sells them around here, it was an extra $500.00 when I bought my boat 2 years ago. Thunderjet in Lewiston (thunderjet.com or thunderjetboats.com one or the other is correct) is where I got mine when I bought my boat. I believe they have a dealer here in Seattle now. Good luck.

Doubletake