please don't hold me responsible if something goes terribly wrong
ok, i really can't say whether you'd be able to use the clips in your current housing without seeing them. if they're mounted to the housing by a screw or a rivot, then you probably can use them. if they're welded or adhered to the housing in some way then you'd probably be better off just buying some. they really are only a buck or two apiece.
IMO, the sturdiest way to do it would be to replace the endcaps with some that you can actually screw down onto the wood. if your housing is anything like my old one, the endcaps are loose inside the housing. if you use just the clips, then the whole bulb and endcaps will be dependent upon just the clips to stay in place. the clips are open ended so one slam of your canopy could result in most or all of your bulbs popping out of the clips. whether they break or not, either way, they're right above your water. this would be grounds for disaster.
look at it this way, you're already saving yourself money since you're using the ballast and bulbs you've got right now. this is the most expensive part of your lighting. so what's 20 bucks on new end-caps that you can screw down and some new clips as well?
if you purchase the mountable end-caps, you'll have those and the clips to hold your lights in place. much safer IMO.
also, although a drill would make things easier, a screw driver and some elbow grease works just as good. HTH