I was wondering how many money vs time you can make from looting all those  ? marks around the main isles(so without main island and those 5 less big isles), the answer:

50k(50.000) money in 7 hours of gameplay

more details: I was on death match on level 27, I had to change 6 boats because of damage, I did sell the loot to anyone with money(I didn't try to sell at biggest price), I got only 600 XP, most of enemies was under my level, I encountered 3 little quests, I had to go to Novigrad to find more merchants with money. The procedure I did, get with the boat near the ? mark, get under water, kill all with crossbow, loot, get in boat , save, go to next ? mark on fast boat speed(without fast boat speed, shift key, will take 11 hours to do all). 
Also, if you are over-weight, on foot and out of battle you will walk very slow, but on horse, in the boat, and in combat , even if you are over-weight, your character will move like it have not over-weight problem, so often I call the horse, to move faster on land. 
So, to me it is worth it, even if it is a little boring, in first 112 hours of the game I did 65k of money, in this 7 hours I did 50k, so I think it is good.

just wondering, is your post more of a guide than a question? i mean as a guide, it's pretty good. i did the same but not on deathmarch. it is quite profitable selling to hatori and the boat ride can be quite relaxing (on easier levels)

It is more a info. Hatori will remain without money very fast.

yes but you do have a stash where you can sell to hattori when he does get money. also other variables may inclde difficulty and how profitable he is during the of swords and dumplings quest.

L.E.:
Tested again with the Complete Edition, I was on level 29, biggest difficulty, no mods, but this time I didn't go to sell until I did it all the marks, so:
1391Kg
5 hours gameplay
500xp
49k gold

Conclusion, it still a good business to farm all the marks.