This is a bit like the question "Which
can you throw further - A cannon ball; a cricket ball; or, a feather?"
The answer is, of course, the cricket ball. But perhaps the tricky bit is
Maximum "A" on the optimisation graph
represents the combination of rocket weight with water weight
that gives the greatest amount of time in the air. At maximum
"A", the rocket travels quite a large distance into
the air and therefore takes some time to get back to earth.
This is analogous to the cricket ball - not so heavy that it
is too hard to throw and not so light that it doesn't have
enough inertia to overcome the drag of the atmosphere (how
far can you throw a feather?)
Maximum "B" on the optimisation graph
is analogous to the feather. One thing to notice about its position
on the graph is that there is a lot of water in the rocket to start
with. All of this water gives the rocket a reasonable altitude
(pushing it through the relatively viscous air - viscous relative
to the weight of the rocket that is) and then, because it hardly
weighs anything, it takes a long time to float back down to earth.
The cannon ball? Imagine that off to the right of
the graph somewhere - too heavy to get much height from the thrust
and heavy enough to fall to earth, realitively unimpeded by the drag
from the air.