Whilst at Wat Chai Watthanaram yesterday a smallish speedboat pulled up, photo op, then darted off into the distance towards Bangkok.

Is it possible to rent speedboats (non longtail) in Bangkok or Ayuthaya?

if you are prepared to pay through the nose, wife tried to hire one for work for 4 hours they charged around $1000. To top it off they were unable to find the ships location that they were given the co-ordinates because no one knew how to operate the navigation so the skipper dropped them off at one of the islands and left them there, mind you they had already paid at the pick up point so it was all worthless. They had to threaten legal action to get a partial refund

Welcome to Thai business.

No ethics for farang.

Lucky you weren't drowned and left to float.

I rented a boat and driver (is that the right word?) about 6 years ago on the Chao Phraya. Spent a day with a couple of the fairer sex along with the driver and sped up and down some klongs, zig-zagged rather scarily the main river, visited a couple of the smaller Wat's, went to a couple of markets and ended up buying a crate of beer with food for the return journey. A very enjoyable day but relatively expensive in the region of around 5k baht. Not something I'd do again as the river taxi's are much more fun and a tiny fraction of the price.

And unlike a jet ski, no way to check for the 'damage' you caused.

I have rented a long tail boat from the Taksin pier for 900 baht. Took a tour of some klongs and temples. Maybe almost 4 hrs. Was low season so it was enjoyable and relaxing. Just myself the wife and boy.