Sounds like something has gotten into your water passages. The usual culpret is one or more of the fins from your impellar. Even though you replace the impellar, you still need to get the water passages cleaned out. Follow this procedure to track it down.
1. Pull the water intake screens on the bottom of the lower unit, inspect, and clean. Look for any blockages inside. (It's amazing how often this simple item is overlooked.)
2. Pull off the cover and while the engine is running, thoroughly inspect all water hoses and connections for leaks.
3. Remove the hose to the cooling indicator port and make sure that is clean and free of debris.
4. Pull the t-stat(s) and test in a pot of water on the stove. Your owners manual should tell you what the initial opening temp and full open temps should be. Also, make sure the t-stats open wide enough, per the manual. (For the 225 beginning opening is approx 140F, full open is approx 160F, and open distance is 3 mm.)
4. While the t-stats are removed back flush the water passages using a water hose with as much pressure as you can get. Typically I use a nozzle fully open, tightly wrap a rag around it, seal it into the t-stat opening by hand pressure, and turn the water on full force.
5. There is a water relief port at the bottom of the starboard thermostat housing (usually a recessed hex head bolt.). If you have a water pressure meter and adaptor with a 1/4" BSPT thread, remove the bolt and install the water pressure gauge and take the reading at idle and check against spec's. If it is too low, you likely have an impeller or blockage problem before the port. If it is too high, you likely have a blockage after the port.
6. Hopefully, you will have cleared the blockage. Then drop the lower unit (as shown in the owners manual) and inspect impeller. To be safe, replace the entire water pump. It's not that much more than the impeller kit, and fairly easy to do.
You can download a copy of the owner’s manual free from hondamarine.com.