White smoke with diesels is either steam or atomised, unburnt fuel. If it's while it's running, it's likely steam. A cloud of white smoke at startup is generally fuel, and indicates low compression.
You need to determine if it;s the result of low water flow causing hot running, and therefore 'steaming' water injected into the exhaust, or water passing thru the engine - ie head gasket / split bore etc. Is the engine delivering usual power and running smoothly? Any problems starting? Is fuel consumrtion normal?
The Thornycroft T90 should have a clean exhaust but can give off a bit (not a lot) of black smoke or soot under heavy load. Light blue smoke on start up is perfectly normal. Light blue when hot can be due to a number of reasons, the simplest being retarded injection timing (accompanied by very difficult cold starting): if it's pumping out light blue smoke, you really need to attend to it. On the other hand, depending on your exhaust layout, a bit of white vapour might be normal: my exhaust outlet is just below the water at the stern and invariably a cloud of vapour follows in the lee of the transom. Insufficient sea water supply to the exhaust will increase the vapour level in any boat and could result in you burning through your exhaust hoses and/or overheating the engine. If the engine starts and runs well, and doesn't use much oil or water, you probably only need to check the sea water circuit (and that might be ok too). What boat do you have?
With an exhaust at or (in "cruising" trim) about 3 inches below the waterline - a good bit deeper at six knots. I've a Mk 2 which uses no water, next to no oil, and trails a small cloud always - that's just what you'd expect. The cloud is MUCH bigger if your strainer's blocked, your impeller's shot, slack drive belt, or you've a leak on the suction side of the sea water circuit, but all of these would show up as hot running. The engine should run at about 85 - 90 C.