While most definitions that apply to batteries are common knowledge, their implication relative to practical use or utility of any particular battery chemistry is not always intuitive. For instance...
First, when someone says 'lithium', it is usually shorthand for 'lithium-iron-phosphate' (LiFePO4) which is the most common and affordable lithium battery chemistry. More exotic lithium chemistries exist, but marine applications are exceedingly rare. Unless you're hoping to power your boat with a used Tesla battery pack, a 'lithium-nickel-cobalt-aluminum' (LiNiCoAlO2) discussion is not helpful. And also a poor choice on many levels.
On the other hand, 'lead-acid' is certainly not specific enough to have an intelligent discussion. While minor variations around cathode coatings exist, flooded (FLA), glass mat (AGM), and gel (GEL) all share the same chemistry, albeit in different physical configurations. Each has different operating characteristics, so specifity is required.
The most challenging definition, in my experience, is 'depth-of-discharge' (DoD). This is simply is the percentage of rated battery capacity that you consume in a discharge 'cycle'. And a 'cycle' is variable period of time between when a fully charged battery begins to discharge current, until the subsequent point when said battery is (fully) recharged.
The challenge with DoD and cycle is that these two parameters are highly related in ANY battery, regardless of chemistry. The greater the average DoD, the fewer number of cycles the battery can be recharged before it needs to be replaced. Fortunately, every reputable battery manufacturer publishes DoD vs cycle data in a easy to read chart. Unfortunately, this has led to some interpretations around battery performance that are either misleading or inapplicable in practical use. The 2x capacity rule of 'lead-acid' vs 'lithium' being the most egregious.
Consider a 100Ah FLA, AGM, of GEL battery, rated at 1000 cycles for 50% DoD, fairly typical. If held to only 20% DoD, this same battery may last 1500 cycles. Conversely, if DoD is routinely and significantly greater than 50% DoD, the same battery may only last 400 cycles. In the case of FLA, discharge significantly greater than 50% can have you back at the chandlery in a matter of months! Important to note that AGM and GEL can run down to 80% DoD occasionally without a huge loss in total cycles. This data is widely available, so simply look it up once you have an overall energy budget and decide what chemistry and capacity is right for you.
Other than safety and simplicity, lithium has a clear advantage in both DoD and cycles, typically 90% DoD and 2000+ cycles. However, as noted in a previous post, the 80/20 rule applies in the real world. Most of us will never burn though 2000 cycles in our boats before we move on to the next boat (or no boat!). And the economics per Kwh are undeniable...AGM is twice as expensive as FLA, and lithium is twice as expensive as AGM.
In summary, for a 100Ah battery you may get 90Ah of 'usable' capacity in lithium, compared to 50Ah in lead-acid, and you'll get alot more cycles (and peak discharge current) from lithium. But after factoring in a safety margin for emergency capacity, maybe 50-100%, and taking an clear-eyed look at your energy budget and cruising behavior, it's unlikely that those 40Ah are going to be a deal-breaker. But you needn't take my word for it. Between published data and knowing your own boating behavior, you can come up with a good decision if you take the time to think it through.
