Imo oil temp is more important, a low water temp of 120-140 is o.k. as long as you get the oil temp up high enough to flow, and take care of condensation. I just went out friday in ice cold water, water temp gauge never went over 140* and my oil was right at 210* once I got her warmed up.

Cold water sucks!!!! LOL. The river is pretty cold this time of year. Not anywhere near freezing mind you but, cold as shit. My engine temp (not oil temp) has stayed below 140. The way I understand it, boat engines are built with cooler running temps in mind so they have closer tolerances resulting in running better under the cooler conditions. I don't see how it could cause you any problems unless you are ingesting ice in your pump. That might cause some issues. Shit, the thought of being on water with ice in it sucks all together. God bless SOCAL

A couple schools of thought on the "Jet boat temp" issue , our position is to allow water to flow freely thru the engine block and regulate the volume and pressure (prior to the entry) via bleed off or pressure relief , We build our engines with Marine ("larger") piston to wall clearances to allow for "cold water" cooling (non expanding block ) re "oil" we typically run  15 40 or 20 50 "high additive PKG oils such as Torco or Red line Tom