Well first off- do you own your summoner you're trying to level up? because you can't level up rental cards...only cards/summoners you actually own. You can always rent a summoner with a level high enough to support your cards w/ higher levels though - that's what I'm currently doing, since I used to have lvl 2 summoners, but since I've been renting I now need summoners at higher levels to allow for all my cards to reach their full potential...

As for other good rentals, are you looking to rent more cards within the water splinter? Or just in general? I personally like to have 1 or 2 specific splinters that I have a solid selection of cards that I can pick from, so that I'm not too pigeonholed into just one splinter while battling.

There are tons of good rentals, you just need to figure out which splinters you're looking to find cards for first (and how much DEC you're willing to spend)...then you can start to narrow your list down to what you need/want/like/etc...and then another thing to do is just be on the lookout for any cards that usually are rentals worth a lot of DEC, but are being rented-out for really low DEC at the time - you can definitely stumble-upon a nice hidden gem every now and then..