Batteries
Lithium Iron Phosphate (LiFeP04)
This is the only lithium battery technology which I will endorse, as it is significantly less prone to spontaneous combustion and explosion hazards. Lithium Ion is the more common technology and is NOT suitable for boats. Google "thermal runaway" for details. Here's a really good overview of some of the advantages of using Lithium batteries.
That said, the upfront investment in LiFeP04 batteries, programmable chargers and battery management systems (BMS) can be daunting. For example, even a discounted 100AH pack (equivalent to almost 200AH lead acid..) is still running about $2500 as of this writing (March 2021). For this reason I still have no direct experience with LiFeP04 on my boat and I'm still running old-fashioned FLA (flooded lead acid - see below)
Lead Acid
This is the most commonly used type of battery. They are heavy but relatively inexpensive, easy to obtain and service, and require fairly simple chargning and control systems. But their energy density is poor and they last only a few hundred cycles (varies by type and quality) before they must be replaced.
More than you ever wanted to know about batteries
Very good overview of lead-acid battery technology and maintenance.
There are different types of lead-acid battery:
Flooded wet cells (FLA) - these are the least expensive and can provide the best energy density. They require maintenance and must be installed in an upright position in battery boxes.
Gel Cells - no danger of acid spills, but generally shorter cycle life.
Sealed Lead-Acid (SLA) - no spills, great cycle life, can be mounted in any orientiation - what could be more perfect? But they are pricey and require a specific charge profile
Exotics
Lithium - the darling of the electric car craze - if I were to replace my lead acid with lithium iron phospate (LiFEPO4), my bank would cost over $6,000 - 'nuff said. [update $3000 - which is starting to look not to bad -- the price is dropping rapidly] However, lithium proponents claim up to 2000 charging cycles. Over a long enough span of time and use, lithium may in fact soon turn out to be the cheapest type of battery.
Here's why.
While lithium cost a lot more up front, you really have to measure batteries by their lifetime watt-hours - how much energy they will deliver before they are no longer usable. Because the number of cycles (charge/discharge) are so much higher than lead-acid you won't have to replace them as often - so the total cost of ownership is potentially reduced. Lithium are already only about 2X the price of quality AGM's - and last well over 2X as long even by conservative measures.
The other advantage of lithium is form factor. You can mount them in any position, and they come in individual 3.5V (usually) cells. So you could probably find all sorts of otherwise unusable space in your boat to mount lithium batteries. You can find (very expensive) SLA batteries that are also in individual cells, but again - no real price advantage.
Now...that said - I am not aware of any current eboat conversions running on lithium and it is definitely experimental territory. Lithium cells require sophisticated battery management systems to ensure they are not over or under-charged.
Update - Current Sunshine, one of the boats in the Conversion Example section, is running on Lithium batteries and seems to be working quite well.
[You might consider a small, say 40AH pack for emergencies - these are only about $1000 (designed for scooters) and might just get you home.]
NiCad - generally not suitable - expensive, toxic and prone to early death
NIMH - a good choice - these are the batteries used in the Toyota Prius, but not easy to find in larger "packs". You can find "crashed" Prius packs on eBay for about $500 - but they are not really that great a deal compared to lithium or even better SLA and there is a lot of work to disassemble them and re-configure into the voltage appropriate for eboats.