The likelihood of the Lithium Ion batteries used in most boats catching fire is less likely than that of a gasoline boat bilge filling up with fumes and exploding.

LiFePO4 batteries are considered safer than other lithium-ion batteries because they have excellent thermal and chemical stability, and they don't normally experience thermal runaway. In slower, low-voltage boats, this chemistry is preferred due to the elimination of thermal runaway problems. LiFePo4 batteries will not reach hot enough temperatures to ignite thereby eliminating thermal runaway issues of Lithium Ion batteries.

In larger boats where energy density is a crucial factor, Lithium Ion (NMC) batteries are used.  If a Lithium Ion (NMC) battery is damaged or handled improperly, the stored energy can be released rapidly, causing a fire. However, the amount of damage that would be needed to cause this would be equivalent to a powerboat ripping through the hull and battery casing. Unfortunately, with an accident of this nature, the battery fire would be the least of your worries. 

Additionally, the high-quality batteries used in modern eboats have advanced Battery Management Systems to cool the battery and thermal insulation to combat the risk of thermal runaway.