It's hard to rate a service that is the most horrible experience of your holiday, but the operator can do little to influence that. To cut it short: strong trade winds cause large waves at the ocean, which move the ship like a nutshell in the washing machine, and it all lasts the endless 75 minutes. The serious sea sickness in the most of the passengers is inevitable then.
Fortunately, the crew were very nice and attentive and provided the passengers with sea sickness bags, always ready to replenish them. The reason why I rate 3/5 only is the fact that air conditioning in hermetically closed ship was switched off, which made the situation much worse, the passengers were sweating like hell and feeling even worse.
What you can do: As I tried on the way back from Praslin to Mahé, you should follow these rules:
1. Take the seats as far back as possible
2. Do not eat anything at least 6 hours before your trip
3. Take as many sea sickness pills according to the manufacturer's instructions
Optional: my wife took the medium dose of sea sickness pills only, but together with one pill of Valium, and she was very happy with this measure.