The big thing is to get them strapped down so they don't move, period. Rough water will slam that battery(ies) around and could crack them. A way to catch any spilled acid is good, which a battery box accomplishes. You MUST have terminal covers for both the positive and the negative. That is about it.
If it were me, I would put them in the console, and glass in a hold down along with a lid of some kind. I only use AGM's these days though, and they are far smaller for a more powerful battery. Hawker Odyssey's work well, and cost a fortune. The Optima brand is more cost effective and what I have in my Prius instead of the smaller factory AGM.
A regular battery box with a strap over the lid should have the battery shimmed inside of it to keep movement to a minimum.
Battery box for boats - Libra-Plast AS
Type: Battery Trays and Boxes
All of this below does not apply to outboard engines, but info can be gleaned from it.
The ABYC Standards that apply to battery installation, Section E-10.7, are a bit lengthy. Call ABYC for the full details.
The main points, paraphrased and abbreviated:
Provide for containing electrolyte leakage or spillage (up to complete leakout), and isolate it from any boat structure that can be attacked by the electrolyte, including fastenings that secure the battery.
The battery should not move more than one inch in any direction (even upside down) when 90 lbs. of pulling force (or twice the weight of the battery, whichever is less) is applied for one minute.
Each battery terminal shall be protected from contact with metallic objects.
It may not be mounted above or below a fuel tank, fuel line fitting or fuel filter. Each metallic fuel line or fuel system component within 12 inches of a terminal or above the horizontal plane of the terminal shall be protected with dielectric material.
A venting system or other means shall be provided to permit the discharge of hydrogen gas released by the battery. (This includes plastic battery boxes and sealed batteries. See ABYC Standard H-2, Ventilation)
Batteries shall be charged by means of an automatically controlled device that is capable of applying the current and voltage appropriate to the type of battery being charged.
Coast Guard, DHS §183.425
(2) Horizontally and parallel to the
boat’s center line for a duration of one
minute fore and one minute aft.
(3) Horizontally and perpendicular to
the boat’s center line for a duration of
one minute to starboard and one
minute to port.
(b) Each battery must be installed so
that metallic objects cannot come in
contact with the ungrounded battery
terminals.
(c) Each metallic fuel line and fuel
system component within 12 inches and
above the horizontal plane of the bat-
tery top surface as installed must be
shielded with dielectric material.
(d) Each battery must not be directly
above or below a fuel tank, fuel filter,
or fitting in a fuel line.
(e) A vent system or other means
must be provided to permit the dis-
charge from the boat of hydrogen gas
released by the battery.
(f) [Reserved]
(g) Each battery terminal connector
must not depend on spring tension for
its mechanical connection to the ter-
minal.
[CGD 73–217, 42 FR 5944, Jan. 31, 1977, as
amended by CGD 81–092, 48 FR 55736, Dec. 15,
1983]
§183.425 Conductors: General.
(a) Each conductor must be insu-
lated, stranded copper.
(b) Except for intermittent surges
each conductor must not carry a cur-
rent greater than that specified in
Table 5 for the conductor’s gauge and
temperature rating.
(c) For conductors in engine spaces,
amperages must be corrected by the
appropriate correction factor in note 1
of Table 5.
(d) Each conductor in a multicon-
ductor sheath must be at least a No. 18
AWG conductor.
(e) Each conductor installed sepa-
rately must be at least a No. 16 AWG
conductor.
(f) Each No. 18 AWG conductor in a
multiconductor sheath may not extend
out of the sheath more than 30 inches.
(g) This section does not apply to
communications systems; electronic
navigation equipment; electronic cir-
cuits having a current flow of less than
one ampere; conductors which are to-
tally inside an equipment housing; re-
sistance conductors that control cir-
cuit amperage; high voltage secondary
conductors and terminations that are
in ignition systems; pigtails of less
than seven inches of exposed length
and cranking motor conductors.
