Gush is great late game, but returning two islands to your hand is very problematic early (having that many islands and being able to eat that much tempo loss for just two cards). So I'd make that swap personally.
For card draw, I'm currently running these:
Fact or Fiction - Good synergy with the graveyard, it also offers a lot of potential for capitalizing on mistakes your opponents often make. I love this card.
Treasure Cruise - I'm really into delve right now. And this is basically ancestral recall later in the game. Perfect power level for cube I think because ancestral recall is broken early but is much less broken when you have to exile 7 cards from your graveyard.
Dig Through Time - Solid card. Delve is such goodness in cube.
Frantic Search - This is one of my favorite draw spells of all time. Being card disadvantage rarely matters if you have any graveyard synergies. It's also free and an instant. When you need an answer and you need it right now, this is probably the best card you could be holding besides the actual answer you need.
Ancestral Vision - Walks a fine line for me. T1 it's stellar in any Ux deck. Pretty much nothing I'd rather play. But it's also the worst top deck of all time. And even on T4 it's basically useless. So of all the draw spells I have in my cube, this is the one I am most likely to cut. But one drops are not super plentiful in blue, so it keeps hanging around (I also like the art).
Compulsive Research - Solid card that does a lot of work for control strategies. It's stock keeps rising with all the land focus mechanics too (which in my mind encourage land dense decks -
molten vortex,
magmatic insight, etc).
Thirst For Knowledge - This was better when I ran more artifact support and broken reanimator, but it's still a decent card. Being an instant helps. Discard is a very desirable effect in my cube due to all the graveyard synergies, so even if this ends up just a one for one, it can be worth the 3 mana sometimes.
Cards I'm not currently running but have run before:
Gush - was problematic. The tempo loss makes it unplayable early. Later it's good value (and has sweet combo interactions) but it's low on my list of draw effects in my cube's current configuration. Since the power purge, combo has become almost nonexistent (blink is all that really remains, and I'm chipping away at that deck now too).
Gifts Ungiven - Good but not great. Unless I have a specific combo I'm trying to fetch, FoF is almost always a better deal for the same cost. This card is probably better than Thirst at this point though, so I should probably consider the swap.