There are very few differences between either camera, other than sensor resolution and video capability. They're both from the same series of cameras within Canon's product line.
The Canon EOS 550D was marketed in North America as the Canon EOS Rebel T2i when it was introduced in early 2010.
The Canon EOS 450D was marketed in North America as the Canon EOS Rebel XSi when it was introduced in mid 2008.
Between those two models, there was the Canon EOS 500D/Rebel T1i introduced in early 2009.
The lens pool for both cameras is exactly the same. Any lens that works with the Rebel XSi/450D also works with the Rebel T2i/550d and vice-versa.
The Rebel XSi/450D has a 12.2 MP sensor with a pixel pitch of 5.2µm.
The Rebel T1i/550D has an 18 MP sensor with a pixel pitch of 4.3µm. This is the same basic sensor introduced in the EOS 7D in 2009 and used in various Canon APS-C cameras for much of the decade between 2010-2020.
The Rebel XSi/450D did not have video capability as shipped from Canon. However, Canon's EOS Utility 2 and third party software can be used to output Live View video from the camera that can be recorded by a tethered computer (but not internally in the camera). That's not of much use out in the field shooting birds or surfing, rather than in a studio, though. The Rebel T2i/550D has built-in video capabilities that can record directly to the memory card.
I really want to get into Bird photography and buy various long lenses. (recs on long lenses?)
I also want to delve into Underwater surf photography and purchase an Underwater housing unit. (recs on underwater surf housing units?)
For birding, focal length is pretty much everything. No matter how much you spend, it never seems to be enough. More focal length and wider maximum aperture also tend to get fairly expensive very quickly. In general, you get what you pay for with telephoto zoom lenses.
- Canon's EF 75-300mm f/4-5.6 III is cheap, but probably not even worth what it costs.
- The lowest priced third party 70-300mm f/4-5.6 lenses are not the greatest in the world, but if they're all you can afford they're better than no lens in the 70-300mm focal length range.
- Be sure any third party lens you are considering is up to date with whatever camera you're using. Older third party lenses can have compatibility issues with newer camera bodies that didn't exist when the third party manufacturer reverse engineered their lenses to work with existing cameras. I've got an EF mount Sigma 70-300mm from the 1990s that works fine on my old EOS film bodies, but it locks up any of my EOS digital bodies (the oldest is a Rebel XTi/400D) when the aperture is set to anything other than wide open. For the two cameras in question, any lens model or updated version released after 2010 should be fine.
- For cameras with "cropped" APS-C sensors, such as the two you've mentioned, any of the EF-S 55-250mm lenses can be a good value. They're optically about as good as Canon's consumer grade EF 70-300mm f/4-5.6 lenses, but around half the cost. Of course you give up 50mm at the long end.
For getting in the water with surfers pretty much any lower budget solution will fit both cameras. By the time you're considering a customised housing for either camera that can't be used with the other, you're in a price range much higher than what both cameras combined are worth.
My recommendation would be to use both cameras. Most accessories and all lenses that work with one will also work with the other. The main exception would be the batteries, which do differ between the two camera bodies. Higher quality third party batteries are pretty affordable, though.
- Hang your longest focal length lens on the Rebel T2i/550D and use it for birding and shooting surfers from the beach. The higher 18 MP sensor resolution will allow you to crop a little more than you could with the 12MP Rebel XSi/450D.
- Use the Rebel XSi/450D with an 18-55mm kit lens that came with most Rebel/xx0D camera bodies for the wet stuff. The larger pixel pitch of the 12.2 MP sensor might give you a little bit better low light performance under water. On the other hand, the newer DiG!C 4 image processing chip used in the Rebel T2i/550D may more than make up the difference compared to the older DiG!C III processor used by the Rebel XSi/450D. The difference in DiG!C processing chips would apply mainly to direct-from-camera JPEGs. For raw files, the larger pixels should make a difference when images from both cameras are converted using the same external applications.
If you lose the older body and cheap kit lens due to water damage, it won't sting as bad as losing the newer body and any more expensive lens(es) you might buy.
As far as various versions of Canon's EF-S 18-55mm lenses go, the IS variations tend to have better quality control than the older non-IS versions. (The EF-S 18-55mm f/3.5-5.6 II that came with my Rebel XTi/400D was pretty bad, with a spot left of center that was always soft, no matter what aperture or focus distance was used.) The general consensus is that the IS versions, which are based on a very similar optical formula, were more carefully screened before being allowed to ship from the factory.