John Ciardi in A Second Browser's Dictionary (1983)
does not cite Kronenberger but attributes the full
expression "happy as a clam at high tide" to colonial
times, and explains that "the water then was too high for
clamming. It is simple enough to suppose that the
happiest one can make a clam is by leaving it alone." He
goes on:

"As a random inquiry into the nature of idiom, I
have asked hundreds of people what they
supposed could make a clam happy. Very few, in
fact almost none, knew of the earlier and lengthier
form, yet all understood the clipped form
accurately [as meaning very happy]."


I think there are some lessons here about both clams and
people.

Mark Dorset:

Here is a misunderstanding of what makes a clam happy,
from Edouard Manet, writing to Zacharie Astruc, during
the summer of 1879, when he was spending time at a
hydrotherapy spa in the Parisian suburb of Bellevue,
undergoing treatment for syphilis:

"As you say so well, time is a great healer.
Consequently, I'm counting heavily on it, living like
a clam in the sun when there is any, and as much
as possible in the open air; but even so, the
country has charms only for those who are not
obliged to stay there."

(quoted by Otto Friedrich in Olympia: Paris in
the Age of Manet, page 283)