If you can; try testing your 898c Si unit on someone else’s boat that is running a Humminbird Si sonar unit. Any model will work for this test. You should also test their unit on your boat.
If the problem stays with your boat (and transducer), then remove the transducer and while holding it down in the water; rotate it around so that the left side of the transducer is pointing away from the boat and see if it improves or not. If it improves than your problem is that something was blocking the left Si sonar. If it does not improve, than you have a bad HDSi transducer.