Well a ravenous Bengal tiger would be pretty near the top of the list ... but it is an interesting question.
My first thought was "termites," for the obvious reasons that they eat wood and would therefore not make a positive contribution to my life expectancy. However, this must be weighed against the value of the fungi they cultivate  and their own, yummy, protein-packed little wriggly bodies. Also they don't sting, so provided I am vigilant about them chewing through the bottom of the boat, they would, on reflection make rather agreeable partners on our journey (even if it would be rather shorter for them than for me).
I then thought about some large and vicious primate, such as a chimpanzee or gorilla. While undoubtedly fearsome creatures (and the chimp to be feared more here than the gorilla, given they are omnivores, which means me-vores), I am confident I could fend these off somehow until they submitted to their fatal flaw: the need for sleep. Whereupon I would bash their brains in and eat them. I would also get to wear their skull as a hat for my eventual rescue, which is pretty badass. So not really a bad outcome either.
Now we should consider exotic and venomous snakes and the like. Unpleasant boat-fellows, there can be little disagreement there. However I am on a lifeboat. Which means what? It means, dear friend, oars. Oars which can be used to flip said snakes into the briny deep or better yet - kill them from outside of striking distance and yes - eat them - then make them into a belt to hold up my raggy trousers. Which isn't as badass as wearing a gorilla skull for a hat, but is still pretty stylish if you ask me, given I have been marooned at sea for ages, not locked in a shopping mall overnight.
Now we have the smartarse answers, the ones everyone who has read or seen The Sword in the Stone  come up with: malaria bearing (female ) mosquitoes and/or various bacteria.
Well yes. Very clever. And yes, these would be disagreeable boat-mates to share a voyage with. But still not my candidate for "least want to" ... Also, don't forget the mosquito is stupid and liable to fly off never to be seen again, so result there.
So here is my answer: the animal I would least want to be stuck on a lifeboat with is the candiru .
The which? Well, as you can read at the link referenced, the candiru is a tiny catfish that hates men beyond all all things, God alone knows why. To paraphrase someone or other, if the word "hate" were inscribed on every atom of every molecule of every cell in its tiny hate-saturated body, that would still not be indicative of the smallest iota, the merest infinitesimal fragment of the total hate this devil-spawn holds for the male of the author's species.
For those of you for whom the link is too long, it likes nothing better than to swim up your urethra (assuming you have one), using a stream of urine as a guide, and then extend spines into the interior wall of your penis*, using them to remain in place until the end of its days.
So picture the scene. I am recumbent, in the bottom of the lifeboat, weak from hunger, sunstroke, you name it. Unless this is a Luxury EZ-Boy Excelsior Lifeboat (TM)** - I am imagining the typical "adrift lifeboat" scenario, rainwater slopping around the bilges, my body, odd containers (perhaps labelled "Amazon expedition: Live specimens") bobbing around in the bottom of the boat and so on.
So weak am I, that I can no longer even bestir myself to answer the most basic call of nature and just release my urine where I lie, whereupon the Satanofish scents its prey and the fulfillment of its Dark Destiny ... I shall draw a discrete veil over the rest.
And that, is why the candiru is the animal I would least want to be stuck on a lifeboat with.
* I am aware there is some controversy over whether they actually, really do this. For example (from the wiki article):
Gudger, in 1930, noted there have been several other cases reported wherein the fish entered the vaginal canal, but not a single case of a candiru entering the anus was ever documented.
Well. That is reassuring. Not a single case of it entering the anus and only a handful of reports of it entering the penis (or vagina). Now I do feel better. I tell you what, you go take a leak in the Amazon, skeptic-boy/skeptic-girl, let me know how it works out for you, mmmkay?
** In which case I lock myself in my First Class Xtra SeaCURE (c) stateroom, armed with the complimentary 1-per-guest 12-gauge Mossberg automatic shotgun contained therein and my answer to this question is: "None of them would bother me. My trophy wall is a little bare right now - bring them on!".
More questions on Life of Pi (2012 movie):
- What is so great about the Life of Pi?
- Which animal would you most want to be stuck on a lifeboat with?
- What does the island symbolize in the book The Life of Pi?