If your tanks are clean to begin with, you have a good water
separation/filtration system, and you are using the boat frequently,
you probably do not need any additives. On the other hand they don't
really hurt anything if used as recommended. The biggest risk you face
is that your tanks already have diesel bugs growing on the sides or
bottom of the tank and you do not yet know it. The crisis point occurs
when you go out in rough seas for the first time and suddenly your
filters start clogging up at a very inopportune time as fuel sloshes
around and knocks the tank slime loose. Older boats/older tanks are
particularly prone to this issue but any boat which sits around a lot
and/or never gets run in rough conditions may be suspect.

If you do not already have dual Racors with selector valves and a
vacuum guage, I highly recommend that you make that investment,
particularly on a single engine boat. The vacuum guage will allow you
to monitor the status of your working filter and swap to a new one
before any problems develop. It is worth its weight in gold. A vacuum
guage reading of 5 inches or more is generally regarded as the right
time to change filters.

The other issue you face with a turbo engine is running too slow. You
must run the engine at near maximum cruising speed for at least 20 to
30 minutes a day to prevent carbon accumulation in the turbo. If not,
expensive, premature overhauls will result.