Scatter tubes are not designed to be placed in the water.
They are made of a thick dense card, which also has an semi impervious layer of ink, the result is they are likely to float and will not sink for some considerable time: meaning that you loved ones ashes may end up on shore or floating about.
If you scatter the ashes on the water then place the tube in the water, that is classed as littering: as the urn will take days to break down.
Yes the tubes are biodegradable, they are may of card. However the speed of degradation is dependant on the material and the climatic conditions eg heat, presence of moisture and microbial activity. So in the case of a tube choose either composting or burial where is doesn’t matter that it takes sometime.
In addition scatter tubes would not meet the Royal Navy’s criteria for water burial.
Also I would think looking at Environment Agency, SEPA and Natural Resource Wales literature they too would consider this to be littering, although I doubt due the sensitive nature of the subject they would seek to prosecute.
Finally, I can’t imagine that a loved ones last wish would be to contribute to littering the environment where they are to be laid to rest.
So please if you don’t want to use a water urn – you can use a scatter tube, just don’t place it in the water, empty it and take it home.