First let me put into a paramedics view.
First remember that possible head injury clue slippery rocks (not seeing any injury or fall never now)
Second some of him being tired could just be the effect of the on coming seizure, as there are many types of seizures.
Third recent diving possible DCS
Now to answerer some of your questions!!
With no phone or house ECT available, you have some choice's to make one sure he breathing, with pulse if not take action.
With him breathing and pulse if theyre a way to administer O2 DO SO.
If he's seizing there you have several other problems one weather to move him or not, remember it's better to be paralyzed then dead, or as a last resort
(Life over limb). But my first thought is a seizure or head injury from a fall
Try to make him comfortable, O2 if possible if in no danger i.e. water waves animals heat ect then you have to make a very uncomfortable decision, to leave him get help or not. The key is hear with out help may die with going getting help may die.
So what choices I would stabilize as much as possible and get help. You could thing about moving with the best spinal precautions you can to be honest with out additional help and equipment not going to happen.
This is almost a no win scenario. Its one I wouldn'tt want to be in thats for sure.
Some hints learn the modified Jaw thrust to keep airway open if his swollen tongue.
With that look for obvious injuries even bites or stings, with him not answering or being conscious its going to be extremely hard to access his neurological functions.
And what his past History is for that matter.
My self I would do this, as personally I feel this is no right or wrong answer
One you can play it as safe as possible, stabilize and go get help.
Or you can assist his air way give oxygen if possible and if true seizure most last under min unless it
S status epileptic then have many back to back, but since you dont know his History ECT your starting off in hole to begin with.
You could also choice to move him to the vehicle and drive to help. Man this is a tough call.
Allot depends on conditions location and how well you feel to take care of said patient too. There is a lot I left out due to this can get really ugly as there are many things to consider in treating this patient and many ways to treat this patient if this makes since.