Hum for not being used much that inlet water fitting is in pretty bad shape and it's definitely out of corrosion warranty too.

What year is the boat and is it a Bravo 3 drive? or B 1 or 2? Did the boat sit in dry storage or wet?
The corrosion mess lying just under the exhaust block off plate, is that corrosion build-up on the transom assembly or is it falling debris from removing/cleaning the inlet water fitting area?
Before removing the engine, Was there a black ground wire coming from the main engine ground stud connected to the transom assembly stud where the nut is (with light corrosion around it) / the bonding strap is connected to the gimbal housing??? If the ground wire was missing like it appears to be in the photo, then that would be a highly probable cause of a corrosion issue on these setups.
The owner has 5 choices to make that I'm aware of.
1) Order and install a new gimbal housing which most marine service dealer try hard to push their customers to do basically saying it's the right thing to do to repair this situation.
2) Other dealers will secondly advise to install a thru hull water pickup with maybe a sea strainer too.
3) Or depending on how bad the corrosion really is to the transom assembly, you could offer to just clean up the area/water passage hole the best you can and install the original style water hose with the plastic insert again which will have to be an aftermarket hose very soon as Merc has a new design hose just for this problem.
4) Here's where we just talked about the new Merc design 2 water hose a few days ago,
Getting ready to overhaul my transom assembly. Part questions. . Also see the OEM Merc image below with the plastic tube inside the new design 2 hose. Once again this is if the area is cleanable enough to reuse the hole and I have read where some have tried painting the area back up to help slow down/protect future corrosion issues too.
5) Though afatermarket marine sells the Merc OEM new design 2 water hose like seen in the EBay link below, they also created their own version of the same concept "first" before Mercruiser did. You may want to re-read it's entirety now that you have to deal with one personally Bt.
http://www.amarket.com/BRAVOblock.htm
http://www.ebay.com/itm/New-OEM-MerC...hUXSL6&vxp=mtr
Second item of # 15,
https://www.perfprotech.com/mercury-...70?umodel=1233