OK I'll give my standard disclaimer first- I live in the midwest so the only salt water
I deal with is what I soak my chicken in for a couple hours before it goes on the grill
Anyway, the short answer is that yes you can cut off the battery clips and hardwire it in but there are some good reasons why you should consider putting some sort of terminal there to allow for a means of disconnection.
The longer and more involved answer starts at the panel, and begins by pointing out that the wire used for most inexpensive panels
leaves a bit to be desired and should be replaced with a reasonable quality wire. Another item that you should be aware of is the purpose of the blocking diode. When a solar panel is in sunlight it produces electricity, which can be used to charge a battery. However, when the sun stops shining instead of being an electrical
producer it becomes an electrical
load and if the diode were omitted it would drain the battery it was connected to.
Now, if the panel is hooked directly to the battery and the wiring
gets electrically compromised either through bad insulation
, seawater getting into connectors, or something more drastic like cuts or shorts in the wiring you will still get this backflow unless that diode is installed at the battery end instead of the panel end.
So yes you can snip the clips off and wire it direct, but a much better approach is to remove the diode and wiring, and install it with a quality wire and put the diode between the wire run and the battery.
Whether or not a 5 watt panel should have a charge controller is a topic for a different thread, but good luck and best wishes for your project!