In addition to the comments by Gord, I would add the following:
Look at the exhaust pipe leaving the waterlock, calculate the volume of the pipe up to the point where the
water will flow back into the waterlock (rather than running out the outlet)
Add to this the pipe volume of the exhaust pipe from the
motor (at the point of the water injection) to the waterlock.
When the engine shuts down, all the water in these two pipes will run into the waterlock.
I have just reread Dave Gerrs' book (fort you) which goes into great detail about exhaust system design.
I will précis it here for you, but I suggest that the book "Boat Mechanical Systems Handbook" by McGraw Hill is a really good source of information.
Safe
rule of thumb: The water
lift muffler should have %130 MORE volume than the entire lift line from the canister to the highest point of the exhaust line, (both ways as described above.
It is possible to fill the waterlock if the engine is hard to start, allowing water to back up and into the engine. For this reason fit a larger water lift rather than a smaller one (if space allows) A drain off point is a good idea if the engine is hard to start, you can drain off water that might otherwise threaten to flood the engine.
Very long lift runs increase back pressure. Avoid lift runs greater than 30 times the exhaust line diameter.