Is AVCHD your requirement?
u can get so many other options but recording at h.264 .mp4
GoPro / Dji Osmo Action comes to mind.
* then convert to AVCHD

No need to worry about autofocus with these.
Angle of view is wider , can adjust the fisheye or de-fisheyed view.
Also if shot in 4k, can further crop.

Else, find a good phone with IP waterproof rating, i only had experience with Samsung s5/s7/s10e.
i would vouch for those. convenience too just bring your phone and not additional camera.

This post has been edited by mastering89: Jul 14 2019, 08:55 PM

Sub rm1k , probably can get an older version of GoPro. Hero 5 or Hero 6
or Hero 7 (not black one)

But if u can fork a little bit more, lets say rm1.5k Gopro Hero 7 black or DJI Osmo Action would be best & will last you for a very long time.
Mainly for the stabilization feature.
Its really2 good

Yeah AVCHD has smaller bitrate file.
Not sure if can change bitrate on gopro, but on dji action cannot.
On phone, u can buy 3rd party app like Filmic Pro and choose the quality u want.
imho, memory cards & storage are getting cheaper now too haha.
Or later uplaod youtube and delete from your pc/laptop.