Moisture cycling is hard on wood, joints, and veneers, but I dunno how big a problem that might be in this case. As wood swells and shrinks due to MC, it can undergo checking or delamination; drawers and doors may stick; and glue joints can fail. Hardware can come loose as it is levered out, and contact seals may lose weathertightness. Thirty percent minimum RH isn't all that awful, but much depends on your maximum RH relative to that and, crucially, length of each cycle. Wood absorbs and loses moisture slowly, often lagging several months behind ambient conditions.
The only sure way to monitor things is by measuring the internal MC of wood and panel products directly. If you expect, later, the boat interior to spend long periods at 90% humidity, I'd advise no lower than 50% to 60% now. The rate of loss or absorption increases with gradient, as does the potential for damage. If even your wet season is down around 70% RH, you can prolly dehumidify it to 30-35%.
I always give my hardwood at least a month to shake down before milling and joining it, as it may come from states at 80% humidity and Wyoming is currently running ... 17%, according to the meter on the wall. That can represent 6 points internal moisture content in wood products, or roughly 1/4" of shrinkage across a 12" maple board (flatsawn). Yikes!
One last concern: dehumidifiers drawn tons of power, as they are just reefers turned inside out and they cycle constantly. Older ones were pretty solid, but new Box Store units burn up with depressing frequency. Me mum goes thru one every eighteen months in her basement. When the compressors do kick it, they can drawn a mighty surge of amps. Make sure your power cord is sized for 'angry motor having meltdown.'