Shell Island is huge,,,, Yes there is only ONE shower block on Island but it does have at least 20 showers. You may have to que for showers during main season,,, especially morning time. I found taking my shower in the evening much better.
There is also a toilet block. There are Green Porta-loos dotted all over the Island. We like to camp further away from main area in the dunes near the beach so we always take our toilet tent and porta-loo anyways.
My advice is to go there for a day-trip first if possible and take a drive/walk round the island. It costs £5.00 to access the island for the day (per car).
You can not book to camp on Shell, You just turn up. Only time I have seen people turned away was at Bank Holiday weekend as the island was full.
Hope this helps