I would say the brackish side of salty - bracking at the back of the marina near the overflow but definitely salty - the lock lets a lot of water in during free flow
You certainly need bigger engines - also consider the Coastline remote controls which can be retro fitted - why not look at Atlantics too - better value and very similar to the Broom
I have twin 63's and love the fact that they are simple to maintain, bullet proof and ultra reliable. The only issues have been with the retro fitted electronic throttle controllers
Try BILGEX - really breaks the sludge down - then soak it up with mats or if you feel OK about buying them (!) some incontinence pads - they work a treat and are pretty cheap too
I keep hearing the old adage - better to be in port wishing you were at sea than to be out at sea wishing you were in port - not much help when weather catches you out though!
I had a similar issue with a 6KVA set. Specialists wanted to remove for access. Big £££. Local mechanic took a look, diagnosed blocked fuel pump, £12 service kit and 1hr later all up and running. I'd get a 2nd opinion at least.
There's an 8Kt limit in Chichester harbour but also a marked mile (half in reality) - the harbour master gives permission with the right flags to use this at 20Kts for up to 30 minutes - maybe something similar somewhere on the river? Early morning?
I had a similar experience. Switched the solenoid from the other engine to confirm the issue then simply replaced the solenoid. Prices were pretty high most places but I eventually found it for about £200. No problems since touch wood.