Both are new branches... Research oriented. Chose based on ur passion. .r if u are mesmerized by the wonders of ocean go for it... Or by the mystery of genes and biotech go for it..

Dear

Both are challenging research field and need commitment and passion to reach upto PHD but opportunities wise and salary wise I would recommend Biotechnology .
Ocean Engineering having good scope but limited job opening in India if you can go abroad and do it then job senario will be better

All the best