What follows is just my own subjective "take" on the issue... please
don't treat it as gospel.

The sort of issue I'm more interested in discussing are things like what
type of antenna and how best to mount and connect it to the radio.

Broadly-speaking, you'll have a choice of antennas which do, or do
not, require a good ground plane to operate (quarter-wave monopoles
being an example of those which do, and end-fed half-wave antennas
being an example of those which do not). A lot of boat-mount marine
VHF antennas seem to be of the ground-independent type - they can be
mounted up alongside the mast, or connected to a fiberglass hull, and
will still work well.

There's a lot to be said for buying a "designed for boat mounting"
antenna even for shore or vehicle mounting... such antennas will
probably be made to withstand salt-spray corrosion, and will live
longer than an "inland" antenna.

For mounting such on a vehicle... plan to mount it up above the
roofline rather than down at the bumper. For hatchbacks, a trunk-lip
mount can work quite well. Mounting to a roof rack is also a
possibility. Depending on the antenna height and vehicle type, it's
also possible to drill a hole in the roof or side, and install a mount
through the hole (maybe not the best idea if you plan to sell the
vehicle anytime soon). If you use a ground-dependent antenna, the
mount will need to be connected directly to the chassis sheet-metal.

Of
course, I'm interested in the radios if anyone here has experience with
them. I see 25 Watt (which I'm assuming is the max available) units at
the bottom end for just over $100 and others which seem to be similar in
regards to the specs I understand for over $200 and of course the prices
run upward too. Are there aspects I should look for that I need to dig
deeper into the specs to find?

Long-term ruggedness and reliability is an important factor in marine
radios, and it's probably one that you're going to find in the specs.
Reading on-line and magazine reviews, and talking to dealers (and
users) about problems, and the return rate, is probably a good idea.

Standard Horizon, and Icom, seem to be two of the big-name players in
the "commercial grade" marine radio business.

As to specs... one thing to look for is receiver selectivity. In a
"crowded" RF environment (such as the Chesapeake bay and its shores)
there's going to be a very great deal of transmission going on around
you, including full-power (25-watt) transmissions from shore stations
and boats. A strong signal on one channel can saturate a radio's
receiver, and block out weaker signals on other channels
("desensitization"). The better the strong-signal handling (dynamic
range) and adjacent-channel selectivity a radio has, the better it
will be able to receive distant signals in the face of a strong local
transmission.

Some of the higher-end marine mobile radios have a "dual operating
position" feature. You can connect a second microphone (often with
its own built-in controls and display) located well away from the main
radio, and operate the radio from the second location. This can be a
useful feature both on boats (e.g. put the second station up on a
conning tower) and in shore installations.

I haven't seen any marine mobile radios which have detachable
faceplates (i.e. intended for remote mounting).

For example is there some spec on the
internals of the receiver that would make a weakly received signal more
clear than another radio?

Sensitivity, and selectivity are what you're looking for. The
sensitivity number will tend to dominate the performance under true
weak-signal conditions - that is, when there are no strong
transmissions on other channels in your neighborhood. The selectivity
number becomes important when there are other radios transmitting
nearby.

What about the speaker itself? Would it be
better to have an external speaker or is the internal speaker usually ok?

Depends on the radio. Some radios have good internal
speakers... although I suspect this is not terribly easy to achieve if
the radio has to be water-resistant... and some are just terrible.
The more compact the radio, the worse its internal speaker is likely
to sound (I think).

I'd be surprised if most mobile-type marine radios can't hook up to
external speakers, so you can always add one on if the built-in
speaker is not adequate for your needs. Bi-amping and subwoofers are
optional :-)

Other issues:

- Do make sure you buy a radio which has been fully certificated
for operation under FCC Part 80 regulations. Buying radios
intended for other services (land mobile, amateur, etc.) and
reprogramming them for the maritime channels is *not* a good
idea... it's legal to do so, but *not* legal to transmit with such
radios. Big fines are possible if you're caught.

- On the (somewhat noisy) issue of licensing... as I understand it,
you will not need any license at all if all you are going to do is
"monitor" (receive only)... at least, that's true around the
Chesapeake, which is entirely US territory and subject to US
regulations. The moment you transmit, though, the rules are
different. Since you're talking about a shore/vehicle station,
you'd have to have a "private coast" or "marine utility" station
license, and these are only available to a limited category of
people: see 47 CFR 80.501(a) for a list of qualifying categories.
Possibly (a)(8) would apply in your situation: "a person servicing
or supplying vessels other than commercial transport vessels"?