Brings me back to when I changed the holding tank on my prior boat. I've done larger projects but not worse. This was in a very tight space (had to twist my body much of the time and work one-handed because there wasn't shoulder-width room to let me get down low). I had to cut out half the salon floor, and grind the edges (which made an awful mess inside the boat.

The holding tank was stainless, and stainless can stand up to only so many years before corrosion takes its toll.

The Bosch Multitool was a fantastic find for this job. It allowed me to get VERY close to the edges with great control and the thin cut minimized dust. After tank removed it looked like this:

Unfortunately I still needed to use the grinder inside the boat to get the edges and interior surface flush, then sanding was also required.

This is after painting. (I used Benjamin Moore exterior-grade porch and floor paint -- it was great to be able to custom match the color to the boat's interior liner shade).

I smoothed and finished the edges of the floor and put it back in using a piano hinge on one edge. (The other side was supported by a strong aluminum L-channel, not yet installed in this image). This became usable bilge storage, we called it "the wine cellar":

This shows the floor closed after the project was finished.

The new holding tank went forward beneath the v-berth. All connections at top.