Contrary to popular belief, posing in a swimsuit doesn't always mean you have to do overtly sexual poses. In fact- if you are trying to be a successful fashion model, I would probably avoid the whole pout and pushing boobs together posing entirely.
There are ways to pose in a swimsuit that doesn't exude sexuality but can be fun and flirty. After all, you are targeting a woman to purchase the swimwear not a man. There are bathing suit styles made for teens as well as adults and when it comes to shooting swimwear, your poses should reflect the target audience. Ideally, the swimwear should be shot on location- poolside or beach, but studio images work wonders too for detailing the outfit.
There are different types of poses you can do when sporting a bathing suit, namely a bikini. There are also different types of bikinis and other two-pieces as well so also keep that in mind. Obviously you would pose differently in a string bikini than you would a two-piece that has boy short bottoms and a simple top. See the suggested images.
Like any other type of shoot, the poses you do for swimwear should be natural, graceful and simple. Complicated poses tend to make you look awkward often simple poses can look stunning if shot the right way.
Remember when shooting swimwear shots for your portfolio/agent, keep it PG, regardless of what age you are. Not all clients want a sexed-up model in a swimsuit the purpose of the image is to show off your body without you being naked. Also, international clients have different perspectives on what is considered acceptable so it’s best to play it safe and not offend. For portfolio updates, make sure that the poses you do are safe and appropriate enough for your agent to submit to any client without offending them. Individual and group shots work well. See the suggested pics.
Ultimately, its not relevant to the image being vertical or horizontal, but about it being classy, fashionable. Don’t over accesorise and makeup should be kept to a minimum.