I'm planning whale shark diving from Placencia next spring and I expect it's far enough that no land will be visible. The shop says it's open ocean with no bottom in sight. I believe it's with a DM and group with no chance of getting lost. At least I plan on sticking close.
My second open water dive and my two buddy's first ocean dives were doing the whale shark dives. Fortunately, we all had great buoyancy control. Air management was another issue, though. Still is. Anywho..., I remember being able to see bottom on one of the dives. It was a ways below us, but viz was great. Make sure you're diving within five days of the full moon. We missed seeing the sharks by one day.