Definitely tons and tons of coins - and in truth probably a lot of smaller stuff considering these were local townspeople and not nobles or lords. Ruined paintings and silk drapes, shattered works of art, gilt woodwork and so-on that would have decorated the place. Probably a safe or vault somewhere with the good stuff (coin-wise). As for jewelry and whatnot, I'm sure there's some, but most of it cheap or fake (again, simple townsfolk) and no magical items to speak of.
All in all, I can't imagine there being more than a few thousand in gold down there at most and monetarily wouldn't be worth their while... however, from an adventure standpoint, exploring a sunken wreck in the black depths of a nearly bottomless lake while a literal sea monster is lurking about - all I can say is good times.
The number on it = number that died. There were no survivors. Either everyone went down with the barge or (more likely) Lucretia killed any potential survivors to harvest their greed and prevent any witnesses.
I'd assume that if there had been a safe or other sort of "bank" structure on board Lucretia emptied it before departing/sinking the vessel. Indeed if your players go so far as to ask former patrons and they confirm existence of a safe, that should trigger some questions - if the sinking was an accident why is the safe looted?
I'd also assume Lucretia took anything really valuable and portable before she left. Most of the art/decor, etc. on board should be ruined by spending days/weeks at the bottom of a lake. Most of the treasure on board would be that of patrons and to support the ongoing gambling at the tables. Your discretion on the value of that but based on my limited grasp of Golarian economics, my guess is that would be primarily denominated in silver pieces. If wanted to make it more worth their while yet make sure the challenge was commensurate, one or more of the patrons have arisen as Lacedons (see ghoul.)