If the smoke is brown (black) it is fuel
that didn't burn in the cylinder.
It's common at startup, (but not as common with engines that have only 700 hours on them) and not usually a sign of anything nefarious.
Oil smoke is blue, water
is white, fuel is black (but can look brown).
A quick test: run the boat on one engine (once it's warmed up) at full throttle for 5 minutes and note the maximum rpm
, maximum boat speed, and amount of smoke from the exhaust
. Do the same with the other engine (doing the same route
, and same direction). If the speeds are similar and the smoke is similar, then don't sweat it.
Don't go out of the marina on one engine and back on the other unless you know there's no current
, no wind
and no waves, or you'll get different results.
As a rule
, unless you have a dramatic event (sucking water
into the engine), or filling the tanks
with something other than diesel fuel, your engines will last for thousands of hours.