I will have to buy it online so can't go and try them on.
That is a really bad idea especially for sea swimming, each make and each style are a completely different fit, even between suits supposedly of the same size and style,
for sea swimming you need movement, you need for it to be non rub.
go to a supplier with a really good range of high end suits, try as many on as possible, early morning is the best time,
you need a suit that is easily to get in and out of, you want it not to bag around the back of the shoulder, but also not impede movement, when you have a suit on do lunges, arm stretches,
you don't want any baggy bits, nor do you want it to pull on you in any place.
I am tall, I buy three to four wet suits a year, I would never buy on line, because I am always surprised when trying suits on, the difference in fit,
Getting the fit right makes a huge difference to your enjoyment of the activities.
I have so many suits that have been discarded because of one irritation or another.
It worth making a journey to somewhere with a decent selection, and try all the suits in your size on, it takes a while and remember to do them all the way properly or you really do not have a real idea how it is fitting,
take a friend for help,
a decent rash vest is a must, and a pot of Vaseline for the neck line.
and wet suit sizing is a whole other issue, I have suits from the same manufacture in four different sizes, they make it up as they go along.