Before people start getting carried away with the imagined seriousness of an event or technical fault that may cause an aircraft (ANY aircraft) to divert off the ocean and land ASAP, you may be interested to know that TOILETS are statistically the No. 1 cause.*
Simply - full, blocked, or unserviceable toilets may mean you cannot possibly continue (for obvious reasons), or indeed may mean you cannot even begin the crossing.
Still - despite all the warning signs in every loo, there are people who still insist on putting nappies etc. down there .....
In this case however, clearly there was a fault with the system rather than a blockage. Not a flight safety issue, but a "stopper" nonetheless.
Edit * some aircraft types and/or pax/loo configurations being more susceptible than others.