It's common to switch older motors that originally spec'd straight weight oils to an equivalent multi-weight. i.e.: straight 40 is typically replaced by 15W-40 This is almost always fine as long as the original spec was not non-detergent. Straight 30 replaced by 10W-30 is also common.
However, for an application spec'ing 20W-40, replacing that with 10W-30 is kinda dumb. For light duty applications, that's probably OK but I would be concerned about engines run really hard and/or with a delayed follow-up service (long OCI). In addition to the possibility of physical shearing, there is the issue of fuel dilution thinning the oil even more.
If you are going to use one weight of oil as a one-size-fits-all solution, the no-brainer choice is a 15W-40 HDEO. I don't know how a professional shop could screw this up.
20W-50 might be fine for your marine engine … but then again, the thicker oil may not be helpful and the thicker oil may be a noticeable additional drag/load on the motor. And, more importantly, a 20W-50 PCMO oil lacks the proper additive package to deal with the fuel dilution and high-moisture environment of marine duty.