There is a reason the military divers use jet fins.... Just saying. If you want to swim against currents, you need stiff paddles, long fins are designed to move the most water with the least effort, but they are inherently bad at providing torque. Think diesel engine vs. F1 car. F1's use little tiny engines that rev at super high speeds to provide the max horsepower, which is work over time. More HP=more work done with less effort, they also have horrible torque ratings which means they don't have a lot of power. Diesel engines have very low HP numbers, but astronomical torque numbers, so they require a lot of energy to do the work and can't do as much for the effort required, but if you need to pull something, you need high torque.
Jet fins are like diesel engines, you're not going to be as fast in perfect conditions as the freediving fins and you will have to exert more effort to do it, but if you have to kick through the current, you need short and stuff, not long and floppy. OMS Slipstreams are actually better than jetfins for this since they are a bit stiffer and a lot lighter, but the same basic fin design is the same
