wrote: (clip) Handsawing this
quantity of bolts seems not very attractive to me. (clip)
^^^^^^^^^^^^^^^^^
I would use a portable, hand-held band saw like a Portaband or a Milwaukee.
(A cheapo from Harbor Freight might work, but I don't know whether it would
last through the whole job.) Hold each bolt in a vise, and cut it until it
is easy to break.

Another way would be to use a Saws-all to finish the cuts--the blade is
flexible, so it can cut flush while the saw is held at an angle.

If you can afford a boat, I would think you could easily handle the cost of
a saw. If you don't feel like adding to your toolchest, you could rent a
saw and easily cut the bolts in half a day. Is there a shop where your boat
is docked? Maybe they would cut them for you for a fee.