Well you did say rental!
So guessing you are in the USA where stuff is reasonable prices.
1st choice Canon EF 300mm f/2.8L IS II USM.
I did have the good luck to try it for about 60 shots, shooting powerboats. Sadly not on my camera, but on the owners 5D3. It was awesome, super fast focusing, pictures on his iPad3 looked crystal clear; even when you squeezed it open with 2 fingers. Real miffed as there was no USB port on an iPad3 to get my copies!!!
What shocked me, was the fact I could hold it on a monopod - Absolutly NO BLurr whatsoever. The owner said, it had 4 Stop IS [nb he did the settings think it had 3 types of IS as well]. The owner said he could use it without the Mono pod. ( but I did note it was on an extremely solid, expensive looking Monopod when we arrived .
Only cons:- Photowise no zoom, so on a Quayside there is no way to walk back, you must think ahead about your shot to get good framing, ( pic a target coming towards you well before the kill zone and then all is in frame) maybe not an issue for you, but my experience is with zooms. Therefore some shots turned into detailed on board 'Portraits'. But as the quality was outstanding and it really could freeze action and spray, didnt matter. VIP if your are walking some way, this felt real heavy ( just moving from a Quayside to their 4x4; was a bit of an effort - this guy had a mooring pass, so not much walking ), get full insurance, I think he said they were £5,000 or near that. We also did get some interested looks, from some scruffy types, glad there were 5 of us - it just looks like a, "steal me" lens !
Quick after thought. my glowing recommendation for the lens; should I guess be tempered by the fact my experience is with a 7D, and this lens was mounted on the owners 5D3 [obviously some of my enjoyment should be apportioned to the camera "upgrade effect "as well.] Note my camera is 7D so not in the same league, but it is a perfect one IQ and focusing-wise. Just factor it in! For comparison I do have the 70-300L IS USM F4-5.6. This is a superb lens and may be on a practial point be a better option it will also be cheaper to rent. You would also be able to hand hold, that most of the time. Benefit of the F2.8 is the AF speed, and getting all the shots as the light fell off at the end of a perfect day. Must be nice to be pretty rich!
Canon EOS 5DS R, Canon EF 70-200 F2.8 L Mk II IS USM, Canon EF 70-300 F4-5.6 L IS USM, EF 40mm F2.8 STM , RC6 Remote. Canon STE-3 Radio Flash Controller, Canon 600 EX RT x4 , YN 560 MkII x2 ; Bowens GM500PRO x4 , Bowens Remote Control. Bowens Pulsar TX, RX Radio Transmitter and Reciever Cards. Bowens Constant 530 Streamlights 600w x 4 Sold EOS 5D Mk III, 7D, EF 50mm F1.8, 430 EX Mk II, Bowens GM500Rs x4