The soup I had had too much salt in it, so I added a little hot water to dilute it.
この場合、次のような言い方ができますよ。
ーThe soup I had had too much salt in it, so I added a little hot water to dilute it.
「飲んだスープがしょっぱすぎたので、お湯を足して薄めた」
to have too much salt in ... で「…に塩が入りすぎている」=「塩っからすぎる」
to dilute で「うすめる」
ご参考まで!