The Seaking series generic manual simply shows that you CAN connect two batteries in series on the higher power ESCs used with motors that can run on higher voltages.
Or you may want to use two lower voltage LiPos to spread the weight , e.g. port and starboard.
It doesn't mean you must! 😉
Without knowing what boat and motor you are using it with we can't say what is adequate or not🤔 Or what switch current rating you would need.
I doubt you will ever be using 90A. That is only the max continuous current rating of the ESC.
Check your motor specs and see if it gives the current drawn at maximum efficiency.
That will give you a good guide👍
DON'T FORGET A FUSE BETWEEN BATTERY-(IES) AND ESC!!!
I also have some car blade type circuit breakers (Halfords etc) which switch off automatically in emergency (stalled motor!) or can be used as a manual ON/OFF switch.
I think they go up to about 50A or so, which might be enough for your application.
Are you going racing??
Hope that helps.
Cheers, Doug 😎
PS I get my Hobbywing Quicrun ESCs on a 'fast truck from Krick', last two arrived three days after ordering 😊