Upgrade from 450D to 77D or m4/3

If you're thinking m4/3 / something smaller, but can't love that small, and you're shooting EF-S glass, and you want a step up in low light from your 450D, you should have a (really) hard look at the just announced SL2.

It's hard to beat $549 for about a 2-3 stops ISO advantage over your 450D and it's smaller and lighter (but still has that traditional DSLR "truck" grip) and it's cheap for what it is. You also get video output from your DSLR, good output. It's still 9-point AF like your XSi, but, it has a center cross point which should do wonders for your sports needs.

Even the former SL1, is a good fit at around $300 refurbished too. The grip on the SL2 is better though, and it has even better low light performance then the SL1.

77D will give you better controls and continuous shooting and AF, but, it's bigger/heavier/more expensive.

If you want radically better sports AF, might I recommend the T7i, more bang for your buck. But truth told, if you're using your center point AF (which you should) for your sports needs, the SL2 can do the same job as the 77D/T7i for cheaper/smaller/lighter. Just a thought.

Oh, and consider strongly the kit; the new 18-55 IS STM is significantly better than your 18-55 on your XSi, if you use the stock lens at all. Also if cost is an issue, the SL1 with the stock kit lens is DIRT cheap at $298, out of stock at the moment, but keep checking in for a restock. The SL1 is a pretty big step up from your XSi, too, at half the price of the SL2. You do get what you pay for though, that SL2 gives you alot for double the price. Both excellent options for your boat.