That's what I was thinking, but what about the ratings tests from
Powerboat Reports and Practical Sailor. Are these reputable magazines?
Also, someone told me they have had good performance from a Maxrad
MHB5802 vhf antenna. It is the same type of antenna and claims 1/2
wave, 2.4db with ground plane and unity without ground plane. Would
this be a better choice? I know Maxrad is a more well known
manufacturer.

I don't know anything about those magazines. Does the company advertise in
them ? If so do you think they would print anyting negative about them ?
I have almost never seen any magazine print anything negative in the reviews
for any product that was advertised in them.
Most let anybody write anything that will sell the magazine.

For most antennas of that type it is best to just look at how well they are
put together mechanically..They should all have about the same absolute gain
for the same length. I look for a stiff antenna that will not flop around.
Too many people do not understand that to get gain from an antenna for a
given type it must be so big. To double the gain ( such as going from 3 to
6 db) you must doubel the size , minimum. I assume that for a boat most of
the stations they will be transmitting to will be low altitude antennas on
shore or another boat. You want an antenna that will keep the signal toward
the horizon. The 5/8 wavelength antenna usually does that. They are
usually around 34 to 36 inches in length in that frequency range. The 1/4
wave antenna at about 16 to 20 inches will tend to put much of the signal
toward the sky like a round ball would look laying on the water. The 5/8
antenna will spread it out like you put your hand on the ball and mashed it
flatter.
On land for mobile antennas it depends on the area you are in as to which
antenna will work the best. I have compaired amny kinds of antennas over
the years by switching with friends of mine and looking at signals from
transmitters at all differant directions. There was no clear winner on all
stations.

Ralph Mowery wrote:
"The 1/4 wave vertical at about 16 or 20 inches will tend to put much of
the signal towards the sky like a round ball would look laying in the
water."

The mental image Ralph`s statement creates in my mind is mistaken. The
1/4-wave vertical antenna over a reflecting plane has a pronounced null
at its tip. The pattern is no hemisphere.

See Fig 3 on page 3-5 of the 19th edition of the ARRL Antenna Book for
the vertical radiation pattern of a 1/4-wave vertical antenna over
earth. Over a ground plane antenna at any elevation, the pattern is
similar, that is, there is a pronounced null at its tip.

Best regards, Richard Harrison, KB5WZI