Prop Cost

Cracked shafts in the key way, bad taper fit, props have to be laped fitted to the shaft, even if new. The key is not their to drive the prop.
When your on the hard lap fit your all your props, then they can be changed underwater with no problem. Have re lap evert time you get them back from the prop shop, moving them in recon process burs the bore.

I would never change a prop under water. I would haul out, check all your running gear, retune/repair/rebalance your props in a prop shop, do a crack dye test (easy check for stress fractures using a spray can of special dye) and then make sure they “blue fit” them back on the shaft (use a blue die to make sure you have perfect alignment fit of your props back on your shaft). These things are your lifeline! All the best.

I would pull the props in the water and have them taken to a prop shop. I've had lots of props changed in the water without issue, have also had lots of props changed at yards too. You mentioned having 6v53's. If they're TI's, cruise should be 2400 rpms and top at 2800 rpms. Always running Detroits way under cruise RPM does them no favors. I have run Detroits at 1000 rpms for long periods of time for fuel range/efficiency but every 6 hours run them up to rated cruise for 30 minutes. Did they make WOT rpm's during the survey? I'm assuming the bottom was clean when surveyed as you mentioned the props were perfect. If they did not make top RPM's at that time, now is the time to remove some pitch. Given the age of your vessel and size of the props, it should be easy to find a reconditioned used set from a prop shop.