I recently replaced both of my 19s group 27 deep cycle batteries with a single 100 ah lithium battery. Not only was it competitively priced, but it weighs less than 25lbs, making much easier to install. Last Monday I turned on the clamp on oscillating fan, the cheap ones from wallyworld, and just now remembered it was still running. That's about 120 hours of continuous use. Placing a voltage meter on the battery shows it is still holding 12.45 volts. I was intending on purchasing another to have 2 batteries on board, but now I don't believe it's necessary.
https://www.amazon.com/LGECOLFP-LiFePO4-Rechargeable-Phosphate-Lighting/dp/B0CNSV316L?pd_rd_w=8ijYV&content-id=amzn1.sym.8847ffa2-364c-4e21-8786-46199e4e9fff&pf_rd_p=8847ffa2-364c-4e21-8786-46199e4e9fff&pf_rd_r=1T9TWYAJSN1NG1AVPA4T&pd_rd_wg=lAUSQ&pd_rd_r=3f52dd6d-b133-42de-a567-c18ecaf9382e&pd_rd_i=B0CNSV316L&psc=1&ref_=pd_bap_m_grid_dv_rp_0_3_t¹ (https://www.amazon.com/LGECOLFP-LiFePO4-Rechargeable-Phosphate-Lighting/dp/B0CNSV316L?pd_rd_w=8ijYV&content-id=amzn1.sym.8847ffa2-364c-4e21-8786-46199e4e9fff&pf_rd_p=8847ffa2-364c-4e21-8786-46199e4e9fff&pf_rd_r=1T9TWYAJSN1NG1AVPA4T&pd_rd_wg=lAUSQ&pd_rd_r=3f52dd6d-b133-42de-a567-c18ecaf9382e&pd_rd_i=B0CNSV316L&psc=1&ref_=pd_bap_m_grid_dv_rp_0_3_t%C2%B9)
I added the same size lithium battery to my boat as you Carl. And I did the same thing in thinking I may want to add a second battery. I too can see that will likely never be necessary.
LiFePO4 batteries are great but be aware that they can cause your outboard alternator (or stator) to fail.
https://www.youtube.com/watch?v=pnqtzmE88uA
Interesting video. The o/b I'm currently using doesn't have an alternator, but I'll keep that in mind.
My outboard doesn't have an alternator either. I do know folks that are charging lithium batteries from their altenators but not without the proper components in-between.