I agree the overal idea is efficient. Generating power with explosions is already finetuned within combustion engines. End its highly poluting, more then coal, gas etc..

But your "hole in the ground + plug" idea would work perfectly if its near shore and you have the water which comes in with the tide to make the giant plug go up and down.

Just research where the tide difference is the highest (and has a market for selling electicity), how you want to convert the movement to energy, the initial starting cost, maintanace coste. And expected power production.
then you you will have your answer if its feasible or not