I have heard of whale sharks in lots of places. I guess for a "guaranteed" trip, you'd have to do a live-aboard in the Galapagos. I went in late October of 2009, and we saw about 23-25 of them in two days on Wolf and Darwin. On one single dive we saw five (and we dove Darwin 4 times, and Wolf 3 times). It was beyond impressive (even despite the horrendous currents).
In the Galapagos, we do the "Galapagos grab" (my name for it, anyway). You go out to the site, and often the current is ripping all around you (especially at Darwin). The whole group back rolls off the panga (the hard-bottomed dinghy). The photographers make a quick dash up to get their gear, then everyone swims like mad for the bottom, for something to grab on to. Once everyone gets settled in, it is a waiting game. You are hanging on to the rocks (they're volcanic, and the current is so strong the nothing is growing there), and just waiting. Basically, you are "watching the show go by", as the drop off is severe off the wall, and the critters (and I mean LOTS of them) are cruising by. Often, you roll off near a school of hammerheads, and drop down.
So you wait. The viz isn't all that great. But when a whale shark comes, it kind of like the sun going down. There is a big, BIG, darker spot in the water. The divers then swim like mad to intercept it. You can't touch, and these things are awesome. We had one come up by our panga, and actually rubbed along side of it (was it trying to mate with our boat???). It turned around, and came right up to me (l had to back up, and it came six inches away from me). You are not allowed to touch them (and shouldn't, anyway, even if you could). They are incredibly gentle (thank goodness) and gracefull in their size.
And, yes, I would do the trip again. October and November are the "for sure" months in the Galapagos. You have to do a live-aboard to get to Darwin and Wolf, though. It is not for the feint of heart, as if you mess up, there are no helicopters to come rescue you, and the trip back to the area where there is a chamber is 19 hours, one way.
Below is one of my shots. Hard to guesstimate, but we had one dive where there were divers all up and down the whale shark. The guide thought it was 14-15 meters, and one of the largest he's seen (and he was born on the islands).
