its the lower flex on a deep swivel more than anything, address the lower part of swivel flexing should be solved. always have some backing underneath swivel could be part of the swivel itself, decent core and washers, etc.
I thru bolted all my swivels. meaning a lower brace on the swivel is bolted thru. have 1 swivel on bow. 4 on stern (2 deep and 2 shallow) was for trolling/planer/kite really only need 2. got silly. 2 midships swivels are bolted thru the inner liner and out hull side. had to make some small angled pieces of starboard to handle the liner angle. bow swivel has a big piece of coosa/penske transom core stuff glassed up underneath, the bolts go thru the inner lip just below where the hand rail sits at your knees. once you get over the fact drilling thru the hull liner and out hull sides it is an easy job. use nylon washers on outside of hull...measure 10X. I am a middling diy home carpenter, but this was relatively easy.
prob easier to take photos. pm me if want and can send over email.
few many? years back couple kids just about ripped the side of their seacraft off fighting a giant off the cape. talked that over with Mike@goose a few years past he had a seacraft (maybe still does).
put the 130 in the swivel, set the drag to 40-45 send your buddy running out in the driveway at a right angle to the midship swivel
after thru bolting...problem solved!
oh, and i have a slew of the varius 4-6" long length 316ss 1/4-20 bolts, cuz have to buy in bulk. gathering dust in my garage. down the cape.