Do you want you swim briefs to be entirely see through the moment you drop your shorts or your towel, or do you wish to safely and decently enter the water and reveal all when you come out ?
Your other choice is do you want your transparent swim briefs to be white and so completely transparent or lightly coloured so you get a hint of colour eg Aussibum Ice Blue ?
In your search for ultimate transparency you'll go from thin unlined light coloured or white spandex towards mesh, maybe something like these:
https://www.ebay.co.uk/itm/UK-Men-Se...MMw:rk:18:pf:0
https://www.ebay.co.uk/itm/Mens-Sexy...zbg:rk:29:pf:0
Would you consider a thong ?
https://www.ebay.co.uk/itm/Sexy-Mens...vOSD VMfEYWhw
With all these see through products the waist and leg elastic that hold the item together are highly visible, framing the area within, unless this is the look you desire have you considered going naked ?