It would be a hard choice, but my choice would be either a .22 LR or .30.  You can carry 100 rounds of .22 LR or .30 very easily.  Either rifle would be a bolt action with good iron sights.

 

The .22 LR can be used on pretty much anything.  Because of its small caliber, shot placement is absolutely critical.  That's why good iron sights are a must have on this rifle.  The .22 doesn't make a lot of noise.  Noise can be a problem in some cases -- if you have to escape and evade. 

 

The .30 rifle is OK also.  Ammo isn't as light as the .22 LR and it does make a lot of noise.  Shot placement is critical, but not in the same sense as the .22 LR. 

 

The old M1917 Enfield is tough as nails and homely as a post.  Not a bad choice.  I'd make sure I'd have one spare part for the Enfield -- an extractor, because Enfields break them (mine did) -- and you can carry it in the butt stock cleaning compartment.

 

I've got an old Ishapore-built (1965) No. 2A1 (a 7.62 NATO version of the British No. 1 Mk III SMLE).  It has a Parker-Hale No. 5A match (aperture) sight fitted.  This would be another tough as nails and homely as a post choice for your island escapade.