I really like what David is suggesting about testing before all of the expensive electronics are in the tube. But the only way to test if the end caps are working is to have the pass through holes epoxied, and obviously one of the caps needs to have the wires going through the epoxy. If possible I am going to be just having wires in the tube without all of the electronics and doing some bucket tests and deeper before putting in electronics. And I think I have read somewhere about having a little kit on hand (DI water, alcohol, etc?) in case the tube floods a little... This stuff is worth thinking about - I think quite a few of us are going to need to do the ptfe "teflon" tape method.
By the way I found some 1/4" wide teflon tape. If I double that over it fits inside the o-ring groove. A few wraps around, put the o-ring back on, and the tap doesnt even show. (I worry about if its exposed it might eventually get caught on something during tube cap removal etc).