But that’s because in the Roman calendar March, May, July & October had 31 days, whereas the other months (including April) had 29 days.
The Ides was a celebration of the middle of the Roman month. If we celebrate it today, arguably it makes more sense that we base the date of the celebration on the length of the modern month.
Which is just another example of the weird (to us) way the Ancients counted. In a month of 29 says, the day exactly in the middle (in the sense that there is an equal number of days preceeding and succeeding) is the 15th; in a month of 31 days, it’s the 16th. The ancients had a weird habit of including the first and last days of a period in the count. It’s similar to the week, which, in Rome, had eight days but was called nundinae, meaning “nine days”. Or to the statement in the gospels that Jesus rose from the dead on the third day after crucifixion, whereas we would count Sunday as the second day after Friday. Apparently, that habit still exists in the Romance languages and is called inclusive counting.
Not seawater, but breathing liquids is a real idea. It has been explored in clinical trials for some medical conditions, and it has featured in various sci-fi movies, notably The Abyss.