[unknown] In the expedition 368 islands and treasure chests can be explored. Not all of them seem to contain lifebuoys, but there are those with 5 and sometimes 10. So for an estimation it should be OK to assume there is an average of 5 lifebuoys per chest or island, so those would sum up to around 1850. If you add those to the 4900 which can be found from the older tasks without, it are 6750. So 600 are still missing for 7350 which could be expected to come with the new event tasks at tuesday. In case we should get a global challenge, the last one had exactly 600 event marker items in three of the five reward chests from the global, so it looks pretty plausible.
Yes, if I am right with this estimation, it is much harder to get the gem rewards from the event pass also this time, the challengers already weren't easy, but my estimation is, to clear the expedition you will need to play around twice as many jobs from the job bored as needed for a prestige chest level 10 in all 3 event weeks left. But noone ever announced you would get something for free for buying the event pass, you always needed to also play the whole event pretty actively. And the Statue of Desaster event showed, you can always trust that there are enough event pass marker items to earn, but PF has no idea how this game is played and how many can really be earned by most of the players.
That's why I already recommended several times to just consider about buying the event pass late in the event when the event tasks are all known and you also know how much you could do. Event passes are available until the very end of the event and you will get all rewards you already qualified for directly.