The following is based on content cut from Red Alert 2 and has not been confirmed by canon sources.
The hydrofoil is a Soviet ship cut from Red Alert 2. It was cut from the game at the late stages of development, and was replaced with the Sea Scorpion.
Although small, hydrofoils can be seen in the Red Alert 2 intro among fleets of dreadnoughts. Along with the corkscrew transport, it proves that the intro was rendered some time before the final game was completed.
A teaser screenshot from September 2000, showing the Howitzer and Hydrofoil
Incomplete intro render. The Hydrofoil is on the bottom right