In addition to pumping out a bit of the fuel/water/trash sitting at the tank''s bottom and also adding *the right amount* (more is not better) of biocide when refueling, try to determine how long the diesel fuel has been in the tank. Often boats that go up for sail have been little used for an extended period prior to their sale - often, that''s how they ended up for sale - the fuel can be quite stale and lose some of its energy value. Unlike gasoline, an additive designed to restore diesel''s (cetane?) full rating is available and inexpensive. WM e.g. carries such an additive. This would be a one-time thing...well, assuming regular use by you.
Good luck with the new boat!