If you have a small facility, you may plan to use your Seawalker primarily before and after training sessions, or on days when horses will not be ridden. This will give a wonderful cooling and anti-inflammatory affect, provide exercise targeting the core muscles and strengthen the hind end and top line. For this, a 15 metre, 6 horse walker with a maximum water depth of 60cm would suffice.
The project cost would be significantly more for an 8 horse, 18 metre diameter Seawalker with a depth of 1.2m. The larger, deeper facility would allow you to replace a high proportion of ridden work, handle 90+ horses a day and tailor water depth programmes for individual horses or groups.
Other options to consider are whether you would prefer an in-ground or above ground track. Financing may be available for above ground equipment in the UK.