I recently went for a 90 min video analysis lesson with a Swim Smooth certified coach. It was awesome, really informative. Relatively pricey, but the best money I think I’ll ever spend on swimming.
Even if you were able to film you own underwater footage the big challenge is analysing what you’re seeing. It’s not as simple as trying to copy the footage of elite swimmers like Ian Thorpe. In fact, one of the biggest faults in my stroke was caused by me trying to swim like the elites (but without their flexibility!).