I think it useful to look at what one considers "overkill" to be and why is it necessarily bad.
Overkill, in the sense of power? I dunno, since I think that it's difficult to have too much power, especially if you're going to be shooting in limited visibility waters. More power may also give you more working distance too. As long as you can adjust the power down on the strobe for shots that do not require a full discharge, I can only see this as being a benefit. If, however, you do not ever anticipate making full use of the capabilities of the strobe - ever - then yes, I would concur that it is "overkill" in that sense as you're paying for something that you will never use. My contention is that in less than ideal conditions, having a good strong strobe will allow you more flexibility, p&s and dSLR alike. I don't think there's anything magical about going from a p&s camera to a dSLR that suddenly makes a requirement for a stronger strobe. However, the features of the DS-125s that I use that were of great benefit when I upgraded were the wired connection and TTL capabilities.
Overkill in the sense of cost? Perhaps. You can certainly get cheaper setups that will certainly do an adequate job. However, I would suggest that if you have any thoughts of upgrading at some point to a more sophisticated system, you'd end up saving money if you get a decent strobe that will last you through at least your next upgrade cycle.