This has moved on a bit has it not?
To be honest, the "diving" aspect is a bit over my head ... despite achieving a compulsory life saving certificate when in training school at the age of approx 21, I can barely swim :-(
However you're saying that rubber has a comfort and grip that the other options don't have.
I'm thinking that a two piece strap is more up my street. Having said that, if you look at the pic you'll see that three out of the five are stainless bracelets that, more or less, came with the watch and the other two are cheapo's that are nothing other than functional (and the Timex is definitely utilitarian: I found it laying in the road in a Devon village years ago and it refuses to die). I'm now wondering if a rubber two piece might be OK. I'd never thought of any of my watches, past and present, as being uncomfortable although they do sometimes slip round my wrist and need to be retrieved occasionally.
BUT some are £2.99 inc. postage, and others are £28.00 The Yellow Dog rubber two piece is £25 I think, probably plus a bit of P&P.
Ongoing food for thought then ....
By the way, unfortunately TF don't do any 18mm rubber straps.