I've been reading some of the floating device threads with great interest as I also have a fear of losing my little Spark in water, compounded by all the reports of flyaway's. But the soloutions seem to carry some unintended consequences.
The floating ideas and contraptions are well intended and serve a good purpose, keeping your drone safe and protecting your investment. But in attempting to protect your investment you may be trading one risk for another.
Another recent and unconnected thread about a flyaway saw someone fail to have a case upheld when it was spotted by the DJI investigators that he had an unofficial attachment on the drone. So you may be risking similar cancelation of a genuine cause for claim if something unofficial is attached at the time of the incident.
I guess its fair for them to argue that it is hard to know how something might be impeding GPS or affecting compass, not to mention how something mounted under the drone might affect the VPS system. If it is trying to rise and get away from something that it thinks is directly under it, but that something is attached, it will just keep rising.
Interestingly DJI store doesn't seem to have ANY attachments that connect to the legs and hang under the drone - even the seemingly harmless leg extenders which actually seem like a good idea. But its possible that even with these extenders on, DJI might have reason or cause to reject a claim based on that unofficial attachment.

This has been listed on the DJI After-Sales Policy. Damage caused by reliability or compatibility issues when using unauthorized third-party parts does not cover by DJI warranty. We don't recommend mounting any unauthorized third-party accessories to the drone. Thank you for your understanding.