Like Unusual Attitude I have a fair bit of subsea experience in oil and gas. £10k would fund some amateurs divng on a known site.
Finding a light aircraft in tidal water with poor-ish visibility and piles of other wreckage on the seabed? You can search for months.
A light aircraft can easily travel several hundred metres before it reaches the seabed, it may well continue to move - until too heavy or restrained by something. The only way to find it is a sidescan survey and then ROV on all the likely objects - of which there are likely to be many.
'Known positions' are actually just vague approximations in this game. Unless there is a team of committed amateurs doing this work the costs of fidnign and recovering the aircrafft would considerably outweigh the new hull cost.