They can't. It's easy to make a "water fort", if you will, but it doesn't do what you want. The trouble is naval units and land units are coded quite differently. This would definitely require SDK modding.
From a purely gameplay perspective (i.e. not taking into account how difficult it would be to code (mostly because I'm not sure how difficult it would be)) definitely improvement. You can already use carefully placed forts to create areas that both land and navel units can go (in worldbuilder) so making it a terrain type wouldn't improve on that at all.
case DOMAIN_LAND:
if (pPlot->isWater() && !canMoveAllTerrain() && [B]!(GC.getImprovementInfo(pPlot->getImprovementType()).isSeaBridge())[/B])
{
if (!pPlot->isCity() || 0 == GC.getDefineINT("LAND_UNITS_CAN_ATTACK_WATER_CITIES"))
{
if (bIgnoreLoad || !isHuman() || plot()->isWater() || !canLoad(pPlot))
{
/* if (fptrb) {
char logtime[9];
_strtime(logtime);
fprintf(fptrb,"[%s]line 2389\n", logtime);
fclose(fptrb);
}*/
return false;
}
}
}
break;// From CvInfos.cpp:
bool CvImprovementInfo::isSeaBridge() const
{
return m_bSeaBridge;
}pXMl->read(m_bSeaBridge)
.. and ..
pXMl->write(m_bSeaBridge)CvImprovementInfo::CvImprovementInfo() :
...
m_iHappiness(0),
m_iPillageGold(0),
m_iImprovementPillage(NO_IMPROVEMENT),
m_iImprovementUpgrade(NO_IMPROVEMENT),
m_bActsAsCity(true),
[B]m_bSeaBridge(true),[/B]
m_bHillsMakesValid(false),
m_bFreshWaterMakesValid(false),
m_bRiverSideMakesValid(false),
m_bNoFreshWater(false),
m_bRequiresFlatlands(false),
m_bRequiresRiverSide(false),
m_bRequiresIrrigation(false),
m_bCarriesIrrigation(false),
m_bRequiresFeature(false),
...
{
}void CvImprovementInfo::read(FDataStreamBase* stream)
{
CvInfoBase::read(stream);
uint uiFlag=0;
stream->Read(&uiFlag); // flag for expansion
stream->Read(&m_iAdvancedStartCost);
stream->Read(&m_iAdvancedStartCostIncrease);
stream->Read(&m_iTilesPerGoody);
stream->Read(&m_iGoodyUniqueRange);
stream->Read(&m_iFeatureGrowthProbability);
stream->Read(&m_iUpgradeTime);
stream->Read(&m_iAirBombDefense);
stream->Read(&m_iDefenseModifier);
stream->Read(&m_iHappiness);
stream->Read(&m_iPillageGold);
stream->Read(&m_iImprovementPillage);
stream->Read(&m_iImprovementUpgrade);
stream->Read(&m_bActsAsCity);
[B]stream->Read(&m_bSeaBridge);[/B]
stream->Read(&m_bHillsMakesValid);
stream->Read(&m_bFreshWaterMakesValid);
stream->Read(&m_bRiverSideMakesValid);
stream->Read(&m_bNoFreshWater);
stream->Read(&m_bRequiresFlatlands);
...void CvImprovementInfo::write(FDataStreamBase* stream)
{
CvInfoBase::write(stream);
uint uiFlag=0;
stream->Write(uiFlag); // flag for expansion
stream->Write(m_iAdvancedStartCost);
stream->Write(m_iAdvancedStartCostIncrease);
stream->Write(m_iTilesPerGoody);
stream->Write(m_iGoodyUniqueRange);
stream->Write(m_iFeatureGrowthProbability);
stream->Write(m_iUpgradeTime);
stream->Write(m_iAirBombDefense);
stream->Write(m_iDefenseModifier);
stream->Write(m_iHappiness);
stream->Write(m_iPillageGold);
stream->Write(m_iImprovementPillage);
stream->Write(m_iImprovementUpgrade);
stream->Write(m_bActsAsCity);
[B]stream->Write(m_bSeaBridge);[/B]
stream->Write(m_bHillsMakesValid);
stream->Write(m_bFreshWaterMakesValid);
stream->Write(m_bRiverSideMakesValid);
stream->Write(m_bNoFreshWater);
stream->Write(m_bRequiresFlatlands);
... ...
pXML->GetChildXmlValByName(&m_iAdvancedStartCost, "iAdvancedStartCost");
pXML->GetChildXmlValByName(&m_iAdvancedStartCostIncrease, "iAdvancedStartCostIncrease");
pXML->GetChildXmlValByName(&m_bActsAsCity, "bActsAsCity");
pXML->GetChildXmlValByName(&m_bSeaBridge, "bSeaBridge");
pXML->GetChildXmlValByName(&m_bHillsMakesValid, "bHillsMakesValid");
pXML->GetChildXmlValByName(&m_bFreshWaterMakesValid, "bFreshWaterMakesValid");
pXML->GetChildXmlValByName(&m_bRiverSideMakesValid, "bRiverSideMakesValid");
pXML->GetChildXmlValByName(&m_bNoFreshWater, "bNoFreshWater");
pXML->GetChildXmlValByName(&m_bRequiresFlatlands, "bRequiresFlatlands");
...
