Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (183.99 USD): BG8OFFCOUPON
Coupon (187.99 USD): BG00c906
(CouponsFromChina.com tip: coupon code has to be applied in shop’s cart after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGissam15wts, BG073996, BG369949, BgGoodie9, BGc363fd
(there’s a little possibility that old coupon will work)
Price history and price alert
Price history (for all/most countries) for TD6E 400W 24V RC Booster Underwater Thruster with ESC is here.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).