There was a treaty after the Glorious Revolution between the Dutch and English that stated they would share naval power to combat the French but it was agreed that the two countries would keep a ratio of ships, somewhere along the lines of of 5 English to 3 Dutch ships. Why would the Dutch agree to this if William III was Dutch himself?
According to William III and His Two Navies (found here: https://royalsocietypublishing.org/doi/pdf/10.1098/rsnr.1989.0010) it was the Dutch who proposed it - the land forces had the opposite ratio.
Short version: the Dutch needed the English a LOT more than the English needed the Dutch, and William needed the English too. And the Dutch couldn't afford more (the English subsidized the war for them as it was).
EDIT: Also, per later in the article, that was awfully close to the existing ratio in ships between the two fleets.