ok, maybe a little late to help but here goes.
I developed cracks in my 2nd to last h18 (ok the first also) after hard sealing in SF Bay. These cracks were near the hull to deck joint at the outboard crossarm attachment under the lip formed by the hull deck joint in the little "half moon" area there.
To correct this problem, I sanded and glassed the area by hand from the inside by brail, reaching through the access port. I believe in this case because of the cracks I put some glass in the half moon on the outside also. I WOULD NOT install a 2d access port, but that is just me. I might cut out a nice hole and glass it back in if I really had to though. I then installed Murrys pn 64110000 1a- front crossarm reinforcement kit. The newer boats come stock with these on the front crossarms and chain plates. I put these in both the inboard and outboard locations.
In my opinion, this boat gets most of its strength from the deck, this is a good system to a point, you won't get the kind of deck cracks that you get in the performance boats (Prindles, Nacras) but the boat generally is not as stiff because of the lack of bulkheads. That tubular thing you describe probably has a name for it and some might actually call it a bulkhead. In any case it serves that purpose. This thing is the heart of the boat, and if yours is "kinda soft" it is dead. Buy a used 18 on the Internet for 600 bucks and get someone to ship you the hulls. I briefly toyed with cutting one of these open from the bottom and inserting bulkheads in 2 pieces from the bottom. But I would rather be sailing.
Hopefully your cracks are in the hull, next to the crossarm joint where mine were. (the deck is stiff, the hull is not as much so and the power from the deck needs to be transmitted to the hull a little better). If this is so then a little glass and the chain plate kit should fix you up.
I bought another h18 with no sign of cracks in good shape, and did the same thing before I put it in SF Bay. I glassed the outboard hull down to about 4 inches from the keel in staggered layers of 6 oz cloth with West. In the critical area (the little half mood depression by the lip), I think I have about 6 layers of cloth up by the lip. The width of this layup at the widest point is about 8 inches.
Some might say this is overkill, and who knows this extra pound might put me in the last part of the fleet forever, but I managed to convince myself the boat might be stiffer and maybe even faster- I am pretty good with those kind of rationalizations.
If you don't have the updated chain plates in this thing, a light day in Lake Havasau might break the boat, much less a heavy day in the Gorge.
note- I haven't cut the access ports in the sterns and put the reinforcing plate on the aft crossarms yet. I am still deciding on this. I have the plates, because you get 2 per item, and I didn't realize that.