Is it possible to backup the hinges with steel bar stock on the backside? And also round off all the bolts or use carriage bolts?
It has been done but it is hard to suggest any specific design or changes simply because the designs vary so much. Generally what exists can be strengthened. The fittings, fasteners and structures can and should be replaced or reinforced.
The usual design is a series of washboards that slide down a track and block off the horizontal structure. These then are secured by a hood that slides over them.
Thicker, possibly marine plywood, boards can be substituted. These prevent, or at least slow, any attempt at splitting or breaking them so they can reach in. Thicker boards or plywood, especially if reinforced with woven roving and epoxy, will hold bolts more securely than the typical thin sawn boards if you want to hang a hasp and lock.
The tracks for the sliding hood can usually benefit from being reinforced. Many are well built for wave and wind action but they can be pried up pretty easily. Once lifted an inch or two it is easy to lift out the boards and gain entry. Stronger tracks stoutly fastened, preferably bolted through reinforced materials and backed with plates if possible, might be an option.
Some designs feature louvered saloon doors in front, outside, of the washboards. Many of these are fairly flimsy but the basic design has some merit. It allows convenient access and allows ventilation while keeping rain and spray out. It also allows some privacy and a small measure of security.
Generally the hinges might be lengthened, perhaps substituting a piano style hinge for the typical leafs, using longer and stronger screws, or better still, through bolts backed with large washers or backing plates mounted on a thicker reinforced structure. Thicker and better attached louvers would be a good start. Most you see can be broken out with a quick punch and a bit of finger work. If the lightweight doors saloon doors were replaced by sturdier models the security aspect could be improved. In effect making the louvered doors another layer of security that would have to be overcome before you could even begin on the main entry.
Of course if you use steel your going to want to use a high quality stainless steel. I like bronze. Aluminum bronze if it is a under a lot of stress. It isn't so important in this application, if your hatches tend to be underwater security is the least of your worries, but I like the look and the freedom from fatigue cracking and crevice corrosion.
Allowed to brown and waxed, instead of being kept bright, the maintenance isn't too difficult. A light wipe to remove the green haze and rewaxing, melted in with a torch if need be, and it looks both 'shippy' and right to me.