INSPECTION PORT UPDATE:
The twist-N-seal hatches won't work on the 18 when replacing the original hatches.
I called my Hobie dealer to order them and he gave me the dimensions. The problem is with the outer ring that attaches to the hull via screws. The stock hatches have a ring that is 8" in diameter (the 6" hatch lid screws into this ring). The Twist-N-Seal hatches outer ring is 9" in diameter. As you 18 owners know the hulls are convex on top and the stock hatches are located just behind the front cross bar. The hatches are not convex they are flat. So the hulls were molded in this area to accommodate the flat hatches and it makes is so the hatches are fairly flush with the top of the convex curve. This recessed area will only accept a hatch with a 8" ring. Therefore, I had to just order the standard screw on type replacement hatches with fat bags.
If you had to add a hatch else ware to do some repairs I believe you could use the twist-N-seal hatch since you would be mounting the hatch ring right on top of the convex hull and seating it with silicone to make a flat spot.
82 Hobie 18 with SX rig and seats