Your engine mounts do not need a marine rating, but need to be the correct version for your engine.
The PO of our vessel took it to a new level, and it works very well: The enclosed Panda 10kw generator
[3 cyl Kubota diesel] is mounted [via its factory engine mounts] to a 1/2 inch [painted] steel
plate [of slightly larger footprint] which is mounted to the rails built into the hull
with engine mounts.
The result is the generator is attached [using anti-vibration mounts] to a ~200lb mass of steel
, which is attached to the hull using anti-vibration mounts.
The net effect is very little vibration is conducted into the vessel [above and below water] Likewise for sound [the gen is in the enclosed, soundproofed engine compartment and has a split exhaust
I suspect this sandwiched, isolated mass approach also reduces the load on the anti-vibration mounts- perhaps increasing their longevity. [None show any signs of needing replacement in over 2300 hours of operation. [The boat came from the tropics and has air conditioning
And since a split exhaust
is used [one each above and below the waterline] you cannot detect the gen is running when on deck
[via vibration or sound] unless you cock your ear over the side where the exhaust ports
are and listen for the bubbles it is blowing... When kayaking up to our boat with the gen running, you cannot detect it until within 10-15 ft or so of the side with the exhaust ports
... any further away and there is no discernible sound.
If we ever frequent waters where there are other boats at anchor
, we are prepared to be the type of neighbors we would prefer...
In case this is of interest since you are replacing your mounts.