Die hard Olympus underwater fan here. I have both systems, the E-PL1 and E-620. Yes the E-620 is faster, but the smaller size of the E-PL1 gives it the edge for travel. However, lenses are extremely expensive for the E-PL1, so your system cost may be similar if you get a lot of lenses. Still, people want the smaller system, and the makers are moving in that direction.
The larger E Volt systems are on the way out. Nikon and Canon are just too far ahead with full sized DSLRs. Olympus has stated it will not release any new midrange DSLRs (the E620 is the current model, now 2 years old). It's not clear what they plan beyond the current the pro level E5. If you try to find all the parts for an obsolescent system, it gets challenging. Over time they will become more expensive. By contrast, Olympus and Panasonic are actively developing new models for the micro 4/3 family like the E-PL2 and the amazing Panasonic GH-2, driving on price and performance.
I did not understand what it was the Olympus sales guy told you, but regardless, go with a more current product. You can pick up an Olympus E-PL2 and Olympus PT-EP03 housing inexpensively, or even a Panasonic GH-2 and 10Bar housing for less than a full sized DSLR, and I suspect you would be satisfied with either, depending on your budget and performance goals.