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.

• First question: 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?

• Second (more general) question: This whole sentence seems a bit strange. Does someone have an idea on how to improve it?

Any help is appreciated.

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?