Hi Erez, blame anything you like but for goodness sakes take care of the problem at least so that you don't crumple to the ground when you get out of the Kayak. Yes, sciatic nerve pressure, yes follow Rehd's suggestion to experiment with increasing under thigh support. I'm with you on this one because my legs will fall asleep in 20 minutes in most minimal kayak seats. Here's the solution:

1. Make your seat deep and sculpted so that the MUSCLES AND FAT of your posterior support your sit bones (ischial tuberosities) UP off the bottom of the kayak. No amount of squishiness right under that sciatic nerve will make up for the lack of support to all of your weight. When you're a standing homo sapiens your skeleton is doing the job it was designed for. When you're a kayaking homosapiens your gluts,butt bones,etc. weren't designed for extended sitting. So it is necessary to make a supportive seat,especially around the sides,back,and under the thighs.

2. I've tried carving minicell, adding thermarests everywhere,adding thermarests on carved minicell, and making a glass seat formed on a mold of my butt. The thermarests work great but only if they are also on some kind of tractor shaped seat. The glass seat I made for my self has no cushion and is more comfortable than any of them. One combination that works pretty good is a thermarest seat pad inside a creature comfort seat. Carved minicell is the cheapest and best alternative although the grey fluff that comes from all that carving is a mess.

3. Condition your sitting muscles,in other words sitting upright. Stretch often.

4. I've heard a lot of people benefit by having a brace at the heel like Nicks footrests because the calf muscles don't have to be held in tension as much as when one is pressing on the balls of the feet.

5. A wonderful side benefit to all that comfort is reduced knee pressure when rolling or bracing hard since the heaviest thing in the kayak (you) isn't sliding around. I found it was a lot easier to lean my kayak when the support in the seat became better. lee-numbnomore