The walkthru is correct, the Thieves stash is on the island directly accross from the harbor, where Lares stands, so not sure why, it is not working.
That key fits one of two chest which need a key, other is the key you get from the Thieves Guild after Constotino's ring, the other two chest need to be picked.
Warning there is a possible bugged TRAP, so make sure you at least save before you swim over or before you enter the cave on the island.
It is a tough call with some games like Gothic I spent maybe 15 hours playing my first time wanting to be a Mage.
Was trying to level up as much as possible before joining cause the Attribs cost changes once your join a guild, ie Strength is very expensive once you join the Mages and Mana is very expensive after you join Mercs or Pallys.
So the theory was build up then join, well I even went to chap 2 and returned to join the Mages and could not, you can only Join them in the Chap 1.
So now if I think I would waste 10 hours and have to do a restart to play like I would like to play, when games do not explain things well, I glance at the walkthrus to save me wasted time.
I think I will buy the guide for Oblivion for this reason, of all the questions I see asked, no one has ever asked a Dev what are the benifets and draw backs of just doing the main quest first, in Oblivion?
I think it could be good to complete the main quest, first then you could have the whole living world to experience afterwards and having all the non main quest to do and any Mods added along would seem very cool to me.