About half the kayak anglers that I know in Florida have switched over to lithium.
Some guys are even using the cheap blue bricks that are sold on eBay, putting them in a water-tight container from Walmart, and using them on their FF and live wells.
https://www.ebay.com/itm/12V-DC-Rech...QAAOSw1O9gUHYC
The blue bricks are inconsistent in power and a margin of risk that is beyond my comfort zone. I don't trust that the cheap Battery Management System(BMS) boards that control these batteries will keep them from exploding. The better quality more expensive brands are watertight sealed and have quality short circuit protection.
In SE Florida, most people are fishing with live goggleeyes which can take the 12v water flow on the live well. Some guys are using voltgage reducers to bring the voltage down to 6v on their Live wells.
The Miady LifePO 16ah 12v was for $54.99 on Amazon, and at 4lbs was probably the best deal. A few yakers are using it, but I see it is sold out on Amazon.
I have a friend who is using two 10ah Nocquas to power a 7" Raymarine Element, and the combined 20ah is not enough if he uses the advanced features over the course of a day, but sufficient if he uses only traditional and map.
It is important to note that most lithiums can be discharged up to 90%, while a lead-acid might discharge to 70%, thus one may not need as many Ah when switching to a Lithium.
Some Lithiums have a limited range of 12V output, and actually, drain down to as low as 7.5v when down to 10% of capacity. It is important to check the operating voltages of your Fishfinder to see which batteries are suitable.
I own a FPV Power 17.5ah battery, which works great for my dragonfly7, but insufficient juice for the Element 7" as its voltage drops too low. This battery is falsely noted as weighing 1lb. It is 2lbs, which is still ridiculously light.
Amped Outdoors offers a good variety of Lithiums and has a chart to note the expected life of various batteries. It's worth taking a look at to note the expected capacities and weights of different technologies.
https://ampedoutdoors.com/
As the technology continues to develop, the prices continue to come down, the longer one holds off on switching, the better the deal one will get -- how sweet it is to have more power at half the weight!