Any suggestions on how to prevent soft spots from froming on the hulls (on the side and deck) of a Hobie 18?

Soft spots can be failed lamination or a failure in the actual foam core sheet. Years ago this foam was sliced from large cakes or "bread loafs" of foam. Density and materials mix could vary depending on the area of the bread loaf a foam sheet was cut from. Some soft spots may be caused by this marginal foam or possibly dry lamination. These boats may not handle harsh conditions. Seems there is a higher proportion (from memory) in Texas. Hot conditions and humid conditions may play a role. Keep it dry and cool if possible when in storage. Do not use a dark colored cover for storage. Keep the plugs out and hatches off, but do not allow rain into the ports. Also a good idea when traveling. Be sure the hull vents when sailing. A hull that is too well sealed will have negative pressure in cool water and high pressure inside when heated sitting on a beach. This flexing of the materials is a problem. Another cause is compression on a hull surface. This can compress the foam or fail the lamination over time. You can see that more commonly in a Hobie 16 deck just ahead of the forward pylon. This is a high stress area that is also a step people use getting on and off of the boat. Also areas of the hull that are walked on (decks and sides while righting).

From what I have read, the ultimate cause of delamiantion is voids, weak points in the layup or bond failure (normally to the core). The material flexs around the weak point until it comes apart.

The prevention is in the construction and design. (Or storing it in a dark climate controlled room and never using it.)

What Matt says will slow the process. However, with some rare exceptions, for older Hobies it is more a matter of how long before rather than will they delaminate.

What I did a long time ago was epoxy 1" Carbon tape on the underside of the lip
all around the boat, since it was cracking and leaking.
(If you question the legality of this the 1989 Worlds TheMightyHobie18's where carbon reinforced.)

In addition, As I recall, I took old aluminum rudder pins and West System'ed them
under the lip at all four beam beam attachment points. (Aluminum is 10X fibreglass?)
At first I only did this at the main beam, but then the ream beam area cracked
and the boat would mysteriously fill up with water.

Then with a D/A sander took off the gelcoat in front of the main beam and covered it with
5.7 oz carbon fibre mesh and paint.
Needless to say the boat and deck became incredibly stiff and very little toe in,
and obviously illegal for racing after this mod.

I'm now repairing/rebuilding the front half of an nearly unused but abandoned & delaminated Prindle 15. The inner skin was no longer attached to the foam, due to standing water inside, and being many years uncovered in the Florida sun. I came to the conclusion that if (waterproof) epoxy had been used inside as a last coat, water would NOT have gone through the inner fiberglass skin to destroy most all of the original polyester resin that held things together for a (long?) while. That skin looked like window screen when I pulled it off, despite someone having tried the polyester injection trick with ~50 holeshots in each panel.

OK- YES- this was someone negligence! But I like simple minded examples. Chrysler was worse than GM or Ford when building rust bucket cars in the 50s and 60s: 3 years later, they were smoking, rust-hole-filled wrecks. So my dad and many other dads would buy a new one!
So- Why am I driving a 19 year old Japanese car now that never rusted, looks and runs like new, and not a Chrysler? Am I brain damaged? OK- I will read my previous paragraph again.