Hi @Ailie,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
My suggestion is to use an oxalic acid cleaner such as Diggers Rust And Stain Cleaner. This is a fairly common approach for removing water stains from timber during furniture restorations.
Oxalic acid is a light bleaching agent, so if it is overused or repeatedly applied, it can slightly lighten the treated area, but under normal use, it targets the dark tannin and iron stains caused by water and neutralises them. This is why woodworkers trust it for spot treating water marks without altering the overall tone of the wood.
In terms of process, you'll want to ensure you follow the user guide carefully to ensure you get the best results. Mix the product in a 1:10 ratio, 1 part cleaner to 10 parts water. After mixing, apply it to the whole face of the piece that you are working on and allow it to soak for a minute or two, then use a stiff brush to scrub the surface before washing it off with clean water.
This should greatly reduce the prevalence of the water stains before you move on to sanding and finishing the table.
You might like to check out How To Restore A Timber Table In A Few Easy Steps for some further guidance.
Jacob
