I found the Blast soda can on Remnant under the water by the cliff opposite the Warthog.
1. Load up a Custom Game (Gametype doesn't matter) in Halo 2 Anniversary on Remnant (not Relic Anniversary).
2. Position yourself facing the Warthog spawn by the Archway with the Speed Boost pickup.
3. Turn right towards the coast and follow the large rock on the edge of the map.
4. Look down and around the sea foam at the base of the island and you will see the Blast Soda Can just underneath the surface of the water.Check out my video to find the exact location:
Edit: Thanks to Whaticles for clarifying that this must be done on Remnant, not Relic Anniversary.
Edit 2: After re-watching my own clip about a thousand times and not being happy with it, I have gone back in and re-recorded the video guide to be easier to follow.