This company makes 'baffle balls' which reduce water slopping around in water tanks when travelling.
They may be a really good idea.
https://enduramaxx.co.uk/tanks/accessories-tanks-fittings/baffle-balls/ 
The ‘Baffle Ball System’ improves the stability of vehicles when transporting liquids by road. In the main this refers to liquid tankers or bowsers with horizontal tanks, constructed of any type of material, but is also applicable for any vehicle fitted with a tank used to transport liquids. Baffle balls come in two sizes and are supplied in two halves which require clipping together. When inserted into a tank the balls greatly reduce sloshing, particularly when cornering or on unstable ground, thereby enhancing the safety, stability and ride of the vehicle. Baffle balls are also suitable for use within boat fuel and water tanks.
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