IIRC the label on the console of my Humber says 0.2 bar which is 3psi....
When I first got my boat and was leaving it moored to a pontoon I used to just run it "as it was on the day" which since I left it soft to be on the safe side when the sun came out (big black boarding wear patches which heat up!) meant it was rather wobbly.
Since having it out of the water over the winter and pumping it up each time before I launch, I have to say the boat feels much much better with correctly inflated tubes than when running them a bit soft. Having thought about it a bit I came to the same conclusion as Nos4r2 - that having them too soft puts a lot of stress on certain areas with the tubes flapping around and wrenching at certain points, especially where they go on to the transom which seemed to take a real hammering.. I just wish I had PRV's so I could leave them hard all the time
An ATV quad bike tyre runs about 3psi and it will deform with a firm poke of the finger but only by a couple of inches so the "poke test" is the yardstick I have used when inflating mine recently - hope it is not too far out!
When I deflated mine to do some repairs I reinflated them with the mains compressor in my garage (100psi) but only till they were almost back into their normal shape then finished off with the tube pump to be on the safe side. Didn't half save some time though!
I looked at a tube pressure gauge from Polymarine here
but it was over £30 which seemed a lot to me!