Float it
I have a 170 with a 3HP Yamaha long shaft, just like you. Never had any problem getting the boat off the trailer. If the trailer is completely submerged, will the boat float? Sure. If the trailer is completely dry, will the boat float? Nope. Try somthing in between. You are not putting your trailer deep enough in the water. Don't worry, nothing bad will happen. If you are concerned having the trailer and car taking an unexpected swim, bring along some blocks or stones to put behind the wheels of the car. I always block may car wheels when I launch and trailer the boat, just in case.