A/C Sea is set based on how the seas are, so if they are moderately choppy, you have the right selection of "auto moderate". The A/C Rain should be adjusted as minimally as possible. The higher you turn it up, the more the weaker targets will disappear; 75% seems too high to me. I'd suggest start at 0 and slowly turn it up until the screen becomes good enough to see real targets mixed with rain...it'll still look a little snowy, but if you get rid of all the "snow", then you'll miss other real targets.
Here's what I suggest for tuning up the radar for optimal results:
Go into installation mode again. Try the following settings while out in open water away from the marina:
On the radar display page:
1. Set range to 0.5nm or less
2. Signal Proc to: I REJ ="LOW"; E AVG="OFF"; and E STR="OFF"
3. Press GAIN button and set: Gain=Manual, 10; A/C Sea=Manual, 10; A/C Rain=0; and A/C AT=off
Got to radar setup in the menu
1. Set STC curve="WIDE"
2. Perform "Radar Optimization"
3. When optimization complete, set STC curve back to "Normal"
From Radar Display Page, press gain key and set: Gain="Auto Moderate"; A/C Sea="Auto Moderate"; and A/C Rain=0
See how the picture looks now. That should give you the best results.