Help need advise!!!

So I brought this Tommee Tippee bottle warmer today from a boot sale to go with my bottles and it doesn't have any instructions so I've just looked online and it ses to add water before turning on but it is saying add 10mm of water below the rim of the warmer??? I may be being really dumb but I don't get that? There are no measurements on the warmer and my other question was there gaps in the bottom and it just didn't look safe to pour water straight into there or is it I've taken a pic of the base so you can see hopefully someone will know if it is safe to pour straight in and how much you fill it up to without measurements? Also is this product any good or should I just use good old boiled kettle water in a jug like my mum said to do :') ahahaha thanks in advance x