is this right?

Water alone will not hurt anything.

I suppose if you got a lot of gritty sand up in there that you could wear the pads. Seems like you'd have to be doing an awful lot of braking though.

I don't usually ride in the rain on purpose, but I've been caught out a few times. Nothing bad has ever happened to my pads.

In Florida doubt you'll have an issue present itself in a short period of time, but what do I know. I do know on extremely wet and gritty descents in a full day of resort riding (we're talking a lot of descending here, something on the order of 18,000 ft of descending that day) I've worn a set of very new (like installed a few days before) BB7 metallic pads down to the point of replacement. I've had a case with fairly new v-brake pads wearing completely out in a race in ugly conditions with about 6000' of descending, too.

The Dude at the LBS...

is full of crapola! In extreme conditions you might wear the pads out in a day as Bikenfool noted. But in one ride?! Not bloody likely! Like Johnathan, I don't ride the trails in wet weather intentionally, but have been caught in a rain storm now and then. I've been using BB7s just about since they came out and have yet to get less than a full season out of a set of pads. I ride agressive XC, and tip the scales a 210lbs. So I give my brakes a pretty good work out on a ride. The pads hold up very well. I suppose there are conditions out there that could wear out a set of metalic pads in a single ride. But I'd hate like heck to ride in those conditions!

Good Dirt