Why wait, I can give it to you straight from the horse's mouth so to speak. I worked in the USCG Office of Boating Safety for 20 years.
Actually, if this is an outboard powered boat the USCG regs do not apply.
Sec. 183.501 Applicability.
(a) This subpart applies to all boats that have gasoline engines, except outboard engines, for electrical generation or mechanical power for propulsion.
Having said that it is still smart to follow the regs, as well as the American Boat and Yacht Council Standards, which is what that CG Auxiliarist will tell you.
Yes you can have the battery in the same space as the fuel tank. A battery is not considered a source of ignition. However, if there is a battery switch then it is a source of ignition. So the space needs to be ventilated.
Sec. 183.620 Natural ventilation system.
(a) Except for compartments open to the atmosphere, a natural ventilation system that meets the requirements of Sec. 183.630 must be provided for each compartment in a boat that: (1) Contains a permanently installed gasoline engine;
(2) Has openings between it and a compartment that requires ventilation, where the aggregate area of those openings exceeds 2 percent of the area between the compartments, except as provided in paragraph (c) of this section;
(3) Contains a permanently installed fuel tank and an electrical component that is not ignition protected in accordance with Sec. 183.410(a); (4) Contains a fuel tank that vents into that compartment; or
(5) Contains a non-metallic fuel tank:
(i) With an aggregate permeability rate exceeding 1.2 grams of fuel loss in 24 hours per cubic foot of net compartment volume, or
(ii) If the net compartment volume is less than one cubic foot, having a permeability rate exceeding 1.2 grams of fuel loss in 24 hours.Reference fuel ``C'' at 40 degrees Celsius plus or minus 2 degrees Celsius from ASTM standard D-471-1979 is to be used in determining the permeability rate.
(b) Each supply opening required in Sec. 183.630 must be located on the exterior surface of the boat.
(c) An accommodation compartment above a compartment requiring ventilation that is separated from the compartment requiring ventilation by a deck or other structure is excepted from paragraph (a)(2)
However it is always best to separate fuel from electrical and putting a barrier between the two is good, even if it isn't required.
ABYC Says
10.7.8
Each metallic fuel line and fuel system component within 12 inches (305mm) of the horizontal or vertical plane of the battery surface containing the terminals, as installed, shall be shielded with dielectric material to protect against accidental short-circuiting. See Figure 1.
NOTES:
1. Terminal insulation or battery covers do not provide compliance with this requirement since, during installation or removal of a battery, these protective devices are usually removed in order to connect the cables.
2. Any non-conductive material may be used for shielding as long as it is durable enough to withstand accidental contact by a tool or the battery terminals during servicing, installation or removal
Also you should not have a fuel line running directly over or under a battery. The battery should have some means of containing any electrolyte that may accidentally spill. A battery box would do this or a tray several inches deep. The battery must be secured so it won't move, and if you put it in a box then the box must be secured as well.
floor is covered with a thick EPDM one piece membrane to protect it from acid/fuel spills. All power cables exit through the hole in front of the battery cover, so they will be on one side of the bulkhead, completely separated from the fuel compartment/hoses.
