For a night dive / surface signal use, I would use a color that is more easily seen than white. My suggestion is either cyan (street light green color) or red / orange.
The situation you describe means the most important thing is rugged, simple, shock resistance, and using cells that don't self discharge. IMHO, that means forget R cells and use a primary like a CR123 size or 2 x AA Li enhanced alkaline.
Slight light output droop is not going to be nearly as important as shock resistance, so I would use a resistored setup - skip the driver altogether.
If I were doing this project, I would use a Lux III Red / Orange with a single CR123 cell and a 1 ohm resistor. It is a large die with very low Vf and high current capability - it is even better than the K2 for this application. You can test the R CR123 as well, but it might need 2 ohms.
Photon Fanatic built a special R/O light for me based on some older CNC123 body designs, a buck / boost driver and 2 x 123. (The buck boost aspect was in case I decided to run a Lux V later instead) IIRC it is set for 1 amp. It is VERY visible and with the low Vf runs a heck of a long time. Red / Orange is the same color as a rear tail light and this LED is the only single LED bright enough to run a rear tail light by itself AFAIK.
Even if you decide to go a different path, try bench testing this setup. BTW - wear sunglasses during the test.
