My 2 cents is that drowning, in general, isn't a very good idea... it will be more painful than it's worth in my opinion. Also, I'm not sure where you live but in a lot of places around this time of year the water can be very cold, which will add additional discomfort. Also, I kind of doubt that death is guaranteed from jumping in. I've seen a few piers in my life and they usually don't go THAT far out into the ocean. You may be surprised at what your body can still do when survival instincts kick in, not to mention the fact that the tide may carry you back to shore.
SN seems like a much surer bet, but there are plenty of methods which would be better than drowning... exit bag and gunshot come to mind.
My sympathies for the hopelessness which has brought you to this point. I hope you find a way to get out of pain, in whatever form that relief comes.