The Iona ferry can roll a bit if there is a swell but the crossing only lasts 5-10 minutes and you will be out on deck in the fresh air so I doubt it would be a problem. The Mull ferry is a fairly big boat and it's a reasonably sheltered route so it would take pretty bad weather to upset most people (and the crossing only takes 40 minutes). When are you travelling?
We are traveling in August. Would you recommend buying tickets before we leave or would the day before suffice?
There are 3 ferries to Mull. The Oban to Craignure is the only one you can book in advance and it gets booked up regularly in summer. I wouldn’t risk turning up without booking. In August I’d book a couple of weeks in advance. If you go for the Lochaline or the Kilchoan ferries you can’t book in advance.
If you are taking a car you should book the Oban to Craignure ferry. If you get there earlier or later and there is space on another ferry, your ticket is still valid.
I can be prone to seasickness - I have lost my breakfast on an Islay to Kennacraig crossing and a Barra to Oban crossing - but I have never even felt remotely queasy on the Mull crossing. The majority of it is in very sheltered waters.