I have a 2005 Baja Islander Merc 5.0 w/ Alpha one outdrive. when going river speed and keeping the steering wheel perfectly straight, the boat moves from side to side approximately 20 degrees each way. back and forth, back and forth, yet continues on a straight heading.....whats the problem...Thank you in advance Greg
First thing you need to check is play in the steering. With the engine off go to the back and try to move the drive from side to side. It shouldn't move at all. You will be able to move the steering cable if you try hard enough. You are only looking for play without moving the cable. It will feel sloppy and move back and forth if there is play.
If there is play, you then need to determine WHERE the play is. Probably a worn gimbal ring from not keeping the pinch bolts tightened every year.
I would be very surprised if it was the hull.
I had a similar boat to your Baja, it did the same thing I DID NOT have hydraulic steering, which was the problem, after upgrading and boat running in the mid 70's I added hydraulic steering, no mor problems.
Plus when I was idlling through no wake zones the boat was all over the place, real pain