I also have a DT55.
I've had my wee engine for a few years now - when I bought it the guy said he'd replaced the impeller recently but after a couple of short trips she cut into guardian mode while we were at sea
, I also noticed that the tell-tail was VERY weak.
Replacing the impeller worked a treat! I also spent some time cleaning the water intakes and found all kinds of crap (This engine/rib was always kept on trailer in drive and covered). The very weak tell-tale was down to a blocked pipe - The pipe from the head was blocked solid! Even the water pump was very dirty.
After spending some time cleaning her out she then ran like new!
I think you may have the same sort of problems! Obviously the low water pressure is caused by restricted water flow???
Replace the impeller and give her a good cleaning!