Hello. My name is Tan.
Can you locate where the water is coming from. There are a number of places it can come in on the engine. You could have a cracked engine block, cracked manifold, drain plug out of engine block, bad u-joint bellows, bad shift cable or leaking transom seal. If you can narrow down where the water is coming from it will help with what the problem could be.
Now has the engine been exposed to freezing temperatures? Has the engine been sitting unused for an extended period of time?
When was the last time you had the drive off and the bellows and u-joints inspected?
The more info and history you can give will help with finding your issues.
Now also for the low on power and backfiring this can be water in the fuel, ignition issue with timing or possible water intrusion.
When was the last time you had the engine serviced. Fuel filters, engine oil and engine tune up. The more info you can supply will help with engine run issues.
If you haven't had any service work done on the engine in the last year or 100 hours of use it needs to be done. Now having all this service work done will more than likely find your issue and fix the problems you are having. A good technician will find the causes during these routine servicing. This is the purpose of yearly servicing to prevent issues like you are having. This is called preventive maintenance.
Post back with the answers to my questions and we can resolve you issues.