How many looters is too many? I've got Merfolk Looter + Enclave Cryptologist + Looter il-Kor, and I was thinking of swapping one of them out for Jace.
The thing I've found with delve is that you need exactly zero enablers to help cast your Treasure Cruises and Gurmag Anglers. Given that you're unlikely to have more than two delve cards in your cube deck (there are really only ~7 cubeable delve cards in existence), and that Riptide cubes are almost universally of the high velocity variety, people surprise themselves more often than not by casting something like Tasigur on turn three, getting it bounced, and still having enough graveyard resources to recast it again on turn four.
I wouldn't start playing bad graveyard stockers to help out delve; delve is a man who needs no help.
Out of these three, I'd swap out Merfolk Looter for Jace. It has the least upside of the three you listed.