Hy there,
I would just like to give you my point of view.
First of all take it easy. In diving one of the most important rules is to take it easy. But in that point, i would like to recommend you to don´t start buying gear even before becoming a certified diver.
As Musicgirl was saying, the best you can do, and the best advice is to try different gear and see what makes you feel comfortable underwater.
I do believe that related to Diving Gear there is as many opinions as divers are in this world. And all of them correct.
So part of your training as a diver will be to experience with different gear and setting up slowly slowly your own.
Now, having said that, me personally fins or boots is not the first thing i would recommend you to buy.
I would recommend to buy a mask. I always recommend my students to buy a mask, they understand it easy so far they have to learn mask clearing, nobody likes the feeling in the begging.
So if you plan to keep diving, make sure you have your own mask. One that fits you and that makes you comfortable underwater.
The biggest reason i´ve seen why divers can´t enjoy their dives is due to the mask. It is so sad to be on holidays, travel far away from home and when it comes to diving not being able to enjoy because your mask is nonstop getting water in. And it will not take too much space in your luggage!!
Now, to the description of your pain, from my point of view, what you are describing is clearly a strong , heavy fast swimming.
The same pain you describe is what i would feel if i should dive fast underwater.
Try to slow down your kicks. As I say to my students, swim very very very very very very slowly, and if you doubt, even more slowly. I rather students to don´t swim at all than to swim fast. At least like this they have to hover.
So next time when you are using your fins, try to relax, try to kick and glide, ich means, you kick once, and move gracefully through the water. Then kick again and move gracefully through the water... and so on... kick->glide.... few seconds... kick->glide...
Remember it is not a race, see it as a pleasant walk underwater.
Now, if you make me choose, I use Jetfins. I love them and even though i tryed and own other kind, i only want to dive with them. I tried and kind of like the Mares Avanti 4.
I hope i was usefull. Please let us know how it worked with slow swim or if yougot fins and which ones.
Feel free to contact me if you have more doubts.
Enjoy your OW course, and remember, diving is a slow motion sport. Relax, breath, act
Happy Bubbles
Gery #scubalegends
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