Which acid to choose when designing a world with acid seas? I'm currently writing a narrative and hoping to include a world which is home to a sea of some form of 'nasty' substance (doesn't have to be acid).

The varieties of inhabitants of this world have evolved mostly on the lands and eventually built their cities right up to the shorelines of this acidic sea. (In some cases I may even have cities that float and migrate across it).

I'm imagining some kind of scientific or industrial endeavor they perform whereby they've created ships that can withstand the chemicals (at least in short term) by lining their hulls with a type of ablative shielding.

Also desirable (although not essential) would be for the composition to not be immediately fatal. A carbon life form (or would they likely be silicate?) could fall in and maybe swim for a few minutes and leave with only some semi minor chemical burns. Yet prolonged exposure would most definitely be fatal.

I'm also wondering if the inhabitants could harness this chemical in some imaginative way such as for propulsion or agricultural/industrial means?

My question is, what kind of chemical/cocktail could I use to fit these kinds of parameters?
Coca Cola or a mixture of Coca Cola and Pepsi Cola would probably be a good choice.

It's not immediately fatal for carbon life forms that fall into it. Only prolonged exposure will cause digestion and damage to bones and teeth.

Due to the phosphoric acid ships with iron hulls would withstand corrosion due to passivation through the formation of iron phosphate (also a molluscicide to protect from shipworms).

In agriculture phosphate is also a potent fertilizer.

For propulsion purposes, Cola together with Mentos is a well-known hybrid rocket propellant.