Ive just taken those two jets off the bomb disposal boat. They dont like them. Ive done loads and loads of jet trials for the MoD on all sorts of boats and engines. Most people dont like them. The boat really really needs to be designed to have jets, the engines need to be lifted up by a few inches, so the intake is just in the water. You loose a massive amount of power (about a quater to a third of your power). They are also alot different to control. Like a jet ski you have to have some power on to control a turn. I think the only reason for having a jet is if your operating in very shallow water 90% of the time. That bomb disposal boat will do 26-27knts with the jets and with gearboxes (props i mean) its doing 36-37knts. Thats alot of difference. Also the impellers need quite alot of maintanance if they are worn of get some crap caught in them they loose even more power as the jet will not work efficiently. Also if your operating in shallow water the intakes do get bloked up with seaweed etc and some times need unbloking.
However they are a very good safety feature, especially for the diving teams, as they have people in the water all the time and very good for safety boat work. No one likes to have a prop spinning near them do they.
Or like i think if you have people near your engines in the water really why not just switch the engines off!!!