That exact safety kit was installed in the little aluminum boat that came with my cottage two years ago. The orange plastic is a lot paler -- it could be 10 or 20 years old-- but whistles and rope and such are very resilient. This winter we accidentally left it on the boat and in the spring, there was water in it (probably a freeze-thaw situation) and the flashlight was ruined. We bought a new flashlight from Amazon, where we could choose just what we wanted. The rest of the stuff is fine.
It should not sit in water at the bottom of the boat. Partly because you shouldn't have water at the bottom of the boat, and partly because it's a pain rolling around the bottom of the boat. Plus you're putting a lot of extra reliance on that waterproofness, asking it to endure sitting in water with the seal submerged for hours or days. It's a lot less to ask that it not leak when rain lands on the lid once in a while. Tie it up at the bow. Here's a not-great picture of the lid tied to our boat:

(Don't keep it hanging outside the boat, obviously, it should be in the boat, but hanging from the railing.) When you put the boat away for the winter, take the safety kit off the boat and put it inside the cottage or take it home. (You could either untie the lid from the boat, or unscrew the bottom from the lid and take just the bottom inside. It's a good time to test the flashlight when you do.)
The most important safety items are lifejackets for everyone, worn, not just kicking around in the boat. The kit stuff is for complying with the rest of the rules:
- One appropriately-sized lifejacket or PFD that is approved for use in Canada for each person on board
- A buoyant heaving line (floating rope) that’s at least 15 m long
- A manual propelling device such as a paddle or an anchor with at least 15 m of rope, cable or chain
- A bailer or hand pump to get water out of the boat
- A sound-signalling device such as a whistle to use in the event of an emergency
- A waterproof flashlight or 3 flares
- Navigation lights if you’re going out during the hours of darkness or in conditions with reduced visibility
- A fire extinguisher if the vessel has an inboard motor, a fixed fuel tank or any fuel-burning appliances
- A reboarding device such as a ladder if the boat rises more than 0.5 m out of the water
- A magnetic compass if the boat is over 8 m long or out of sight of navigation markers
The kit provides the rope, the whistle, and the bailer as well as the flashlight. You don't need the last 4 things, so lifejackets, at least one paddle and the kit means you're all set. We also have our paperwork for the boat in there, in a plastic bag. If you get pulled over you can show it to the water police, who will also be counting lifejackets.