Bernd, 

the SN tank (I assume that SN119 is exact the same as SN120) has about 5l fuel (or a cocktail of fuel and water...) at the bottom of the tank that will stay there if you run it empty. I know it because we pumped out all fuel when we had water in the fuel last spring.
The original Volvo water seperator is not enough, we installed a Racor filter with water separator after the volvo water separator and water startet to collect there that just ran through the (almost empty) Volvo water separator.  When the Volvo alarm lamp turned on the Racor was already full with water, and some water was already in the fuel filter installed at the engine. Without the additional Racor we would have been in serious troubles on our way to hawaii. it is a must to install one on the SN if it is not there.
If you have water in your tank (and it is more likely to get water in your tank in the carribean than in the med) it will be a bigger problem if you do not have a good filled tank, the less fuel you have with some nice waves the more the water at the bottom will be mixed with the diesel. I did underestimate this effect.  Simply adding fuel from the jerry cans and filling up the tank made a big difference in the hourly amount of water we had in the water separator.

We used a vacuum pump that can be used for the oil change (with a very thin flexible but stiff hose) to reach the bottom of the tank , not beeing hindered by the baffle-plates.

keeping you tank full is a wise decision

herbert, SN120, Pacific Mexico