I would not use the fuse rating to estimate the power draw from the lights. Fuses
might have been replaced with something way oversized (or undersized) or might simply have been oversized to begin with.
Most conventional interior
light globes would be around 12 watts or so, hence 1 amp each. I did find a couple of 5 watt globes in an old light fitting recently but that was for the red night vision mode.
Most of the navigation lights I have seen had either 12 or 18 watt globes. The only deck
light I have had direct experience of was a 50 watt halogen, so I don't know if that was normal.
I am yet to find a conventional 12 volt incandescant globe that cannot be replaced with an easily purchased LED equivalent, though often the price
of the replacement LED globe is more than the cost of replacing the entire light fitting. I have, despite this, kept a couple of light fittings and swapped them to LED because it is hard to find a nice looking LED light fitting (in my budget
range) that suits an older boat like ours.
Anyway, in the OP's situation, I would start by making a count of all the globes I needed to replace if I were to swap everything to LED. Then if the price
was in budget
I would go that path and then purchase
either a small Sealed Lead Acid battery (10 - 15AH should be adequate for evening races if all lights are LED) and then it would be small enough to bring home with me to charge on the shed bench during the week (assuming a weekend racer
here). The advantage of this option being that no rewiring should be required and all the switches remain where they are currently (and no running round the boat at dusk turning on small battery operated stand alone lights. I've done this, and I found it tricky, though that was on a 20 footer). That being said, those battery operated LED nav lights lasted for years and were still using the original batteries when I sold the boat.
Lithium would be even lighter than the SLA battery, and if cost was not an issue, then it too could be brought home to charge at the end of the day.
That's my suggestion.