[Avodah] kosher watermelons

[Aryeh Stein]

FWIW, see below for R' Heineman's opinion about using watermelon cut
in a supermarket.

(I would venture to guess that those persons who won't swallow
non-flavored pills on Pesach before checking R' Gershon Bess' guide
will not buy sliced watermelon in a supermarket....)

http://www.kosherblog.net/2007/05/03/r-heinemann-recap-part-2/

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CUT FRUIT & FISH

One can assume that varieties of cut fruit sold year-round in a
supermarket or other similar setting are probably cut with a dedicated
knife, leaving the fruit kosher. Seasonal fruits, like watermelon,
though, may be cut with knives also used in other areas, so when
purchasing a watermelon portion, one should trim a small amount off
the edges to maintain kashrut.

Fish, too, may be considered similarly. If, for example, a whole,
kosher fish has had its head and tail removed in a non-kosher fish
market, we may trim away the cut edges with a kosher a knife and use
the fish. (Since scalers can only be used on fish with scales, i.e.
kosher fish, there are no kashrut concerns about the scaler's use on
non-kosher fish.)

Most interestingly, we may assume that kosher fish which is cleaned
and filleted in an (unsupervised) industrial setting is being
manipulated with dedicated equipment only used for such (kosher) fish.
(To learn how this material might apply to your own fish-buying
practices, consult your rabbi.)

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KT,
Aryeh