The answer is MINK.
I arrive at the answer of Mink by starting with the 2-2 bounding
pattern, this is classic for the smaller members of the weasel family.
Next I look at the belly width of 2½"; too small for otter or
fisher, too large for ermine, that narrows it down to long-tailed
weasel, mink, or pine marten. I can tell you there ain't no pine
Hartford, Connecticut, so it's either weasel or mink. Both weasel &
mink are good swimmers, but mink live in the water and only occasionally
come onto land, whereas it's vice-versa for weasel. Photos E1-E5, going
onto land for a short distance and then right back into water, suggest
mink. Also the belly slide suggests mink.
Photos E6-E10 suggest the animal was swimming under the ice of the
pond and hunting crayfish. Weasel is a good swimmer and a good
hunter, but I highly doubt it's a good enough swimmer to hunt under the
ice. That's why I say mink. Anyone have an alternate opinion?
What I have learned from this is that mink can turn up where you
might not expect them - in a stream that drains out of the city dump,
for instance. Mink move around a lot; they don't den at any
particular spot for long, so you have to put in a lot of Dirt Time to
get lucky enough to encounter signs of mink. I have also learned
the powerful cleansing effect of a marsh - in this case, the nasty water
draining out of the city dump is purified by the marsh to the extent
that there is a bounty of fish, crayfish, and clams in the stream, which
attract muskrats, otters, and even mink.