I'm writing about renewable energy sources and I'm having a bit of trouble with this sentence:

A relatively immature, yet promising source are the ocean waves, which are a ceaseless discharge of energy directly on the shoreline.

Some grammar check services say that I should write:

A relatively immature, yet promising source is the ocean waves, which are a ceaseless discharge of energy directly on the shoreline.

I think can understand why this is correct, since "the source" is singular, i should use "is". But it seems a bit weird to use "is" when writing about "the waves". What is the correct form?