How do I safely and cleanly remove sealant from a plastic bath?
You might have to resort to good old fashioned elbow grease.

The sealant should come off the tiles the easiest, both mechanically by scraping with a plastic edge - the flat edge of an adhesive spreader should be suitable, and if necessary chemically (though I'm not 100% certain what you can use) as the glazed surface will hard to damage.

The plastic bath might cause more problems. I'd start with rubbing or scraping with the plastic edge - but take a lot more care. Chemicals are more likely to damage the surface too. You could try using a plastic safe cleaner.