I love the series - Spring is my favorite, it has the most appealing lighting. A very nice mood. Winter is pretty cool too. But man is this a flat landscape! Like the salt flats in Nevada or Utah, hehe.
For the grainy shadows in the water, do you have Soft Shadows enabled? If so, try increasing samples very high and do a crop render to see if this helps (e.g. try samples of 30 or something). If it does help you can tune the setting to try to achieve acceptable render time and quality. If not, then you may be right that it is a limit of the underwater detail rendering, which would only be solved by very high detail settings (e.g. 2 or 3!), which is probably not practical.
- Oshyan